[Android Q&A, Help & Troubleshooting] j4nn:
In my opinion, you would need to get the bootloader unlocked, in order to recover using fastboot.

Or get into EDL mode via testpoint (if the phone does not enter it automatically due to serious boot error).

I've already described details about EDL mode here in post#53:

It seems ratel cell is using general qualcomm test keys, as can be seen from ABL:

Certificate:
Data:
Version: 3 (0x2)
Serial Number: 1 (0x1)
Signature Algorithm: rsassaPss
Hash Algorithm: sha256
Mask Algorithm: mgf1 with sha256
Salt Length: 0x20
Trailer Field: 0xBC (default)
Issuer: C=US, ST=CA, L=SanDiego, OU=CDMA Technologies, O=QUALCOMM, CN=QUALCOMM Attestation CA
Validity
Not Before: Mar 6 04:51:52 2018 GMT
Not After : Mar 1 04:51:52 2038 GMT
Subject: C=US, CN=SecTools Test User, L=San Diego, O=SecTools, ST=California,
OU=01 000000000000001C SW_ID,
OU=02 3002000000600000 HW_ID,
OU=04 0060 OEM_ID,
OU=05 00000088 SW_SIZE,
OU=06 0000 MODEL_ID,
OU=07 0001 SHA256,
OU=03 0000000000000002 DEBUG,
OU=13 0001 IN_USE_SOC_HW_VERSION
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:d5:1c:c7:74:9d:bf:d1:1e:54:13:50:94:08:33:
9e:ba:bf:a2:40:03:fa:e2:87:7d:43:0f:9a:ea:b9:
c5:45:7a:76:d4:35:8b:32:b2:11:67:56:d5:5e:42:
94:b2:5a:8a:37:15:68:39:8a:c8:86:fa:86:bd:26:
5c:1a:b9:2b:9a:96:38:69:40:dc:ea:05:71:90:e2:
50:5b:1d:15:9a:2b:ea:0a:bb:65:88:e5:8a:b1:39:
5c:92:6b:50:a8:7a:ad:fb:e3:29:99:80:9b:e1:05:
6c:77:6f:50:35:2b:dc:6d:a7:91:8c:e5:ed:fa:80:
db:34:74:ac:ab:09:46:eb:b6:d7:b2:61:b0:a5:7c:
7e:1f:84:98:25:49:8d:73:33:8f:a4:d8:6d:6f:64:
a5:12:3f:da:9d:f7:79:3a:3a:06:6a:1c:62:aa:93:
32:6a:c5:d8:c1:2d:64:86:56:de:33:e8:91:26:23:
29:3d:4d:d8:22:a0:bc:a5:8a:75:25:ca:ad:48:1c:
5c:dd:0e:e6:e1:51:86:10:60:05:54:0f:1e:54:a2:
3f:2b:61:81:2c:8e:9b:d1:7e:03:79:5f:a4:65:29:
a1:a4:d7:33:5b:90:52:4d:b8:fb:35:1d:02:19:91:
4f:59:b2:b1:9c:ec:da:cd:dd:e5:ca:3d:81:a0:4d:
80:11
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Authority Key Identifier:
keyid:F9:9B:C5:64:29:04:1E:F9:0B:BA:4B:97:28:9E:2B:84:AC:00:73:D5

X509v3 Basic Constraints:
CA:FALSE, pathlen:0
X509v3 Key Usage:
Digital Signature
Signature Algorithm: rsassaPss
Hash Algorithm: sha256
Mask Algorithm: mgf1 with sha256
Salt Length: 0x20
Trailer Field: 0xBC (default)

39:be:49:fd:42:c8:93:2b:27:fd:b0:a6:b0:fa:31:bb:fa:e6:
23:f3:f2:02:bd:b6:ed:0a:67:fe:a0:b3:0d:d3:04:59:22:72:
e9:8f:f9:f9:2e:d7:c9:6e:3b:d3:ae:cc:b9:fa:18:60:8e:7f:
16:a1:2d:fd:02:7f:cf:0b:cb:ac:02:74:33:ec:ee:35:c8:b1:
b7:f5:ab:03:9a:74:9b:92:cc:bc:18:51:dd:7d:cb:e0:f6:dd:
f9:76:67:f5:b7:ce:04:b2:dd:7d:94:27:3b:5f:40:2f:0b:11:
35:a3:fa:12:c3:49:a9:4d:47:19:6f:1b:fe:92:35:e7:53:6c:
c8:86:44:d4:b0:0c:8e:df:07:63:50:4e:bc:6c:17:ce:79:11:
9d:1c:39:b2:fb:c2:ac:b0:89:4e:b1:7e:b1:73:58:d2:ad:3b:
79:bf:37:3d:b7:fe:c8:bd:d2:99:ba:36:10:58:8e:8f:fc:be:
91:09:ce:5d:8c:58:80:54:8a:ac:ee:9c:2f:a9:5a:91:b8:9f:
f8:7c:84:38:bf:ae:5c:88:85:f1:85:2e:7f:d4:f6:63:b9:30:
e9:cb:2b:8b:b4:d5:40:41:33:73:e9:72:c7:ce:d6:37:16:ab:
d5:e5:9f:04:cd:ab:55:2e:c8:26:9d:5d:e5:03:d0:af:0f:e3:
c0:34:0a:0c

Certificate:
Data:
Version: 3 (0x2)
Serial Number: 5 (0x5)
Signature Algorithm: sha256WithRSAEncryption
Issuer: C=US, ST=California, L=San Diego,
OU=General Use Test Key (for testing 13 only),
OU=CDMA Technologies,
O=QUALCOMM,
CN=QCT Root CA 1
Validity
Not Before: Nov 22 06:26:53 2017 GMT
Not After : Nov 17 06:26:53 2037 GMT
Subject: C=US, ST=CA, L=SanDiego, OU=CDMA Technologies, O=QUALCOMM, CN=QUALCOMM Attestation CA
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:e3:d9:79:a0:82:14:8a:6f:5f:b9:62:11:a3:38:
85:3f:75:7e:f3:b9:a0:5f:61:44:c0:4c:1b:48:5a:
70:d4:25:c1:8d:9e:50:bb:57:97:92:f9:24:5c:c2:
58:0d:bf:26:6c:4b:27:1f:fd:e3:23:7d:43:a3:74:
68:33:eb:ec:50:bd:87:46:18:7a:07:00:be:a9:8f:
ec:65:b6:5f:27:b1:10:05:48:e9:65:0a:ca:fa:b7:
e8:72:71:21:56:3a:e2:10:54:ab:ff:4f:e2:a6:5b:
99:96:b4:3b:b5:b3:4a:0a:b2:36:18:3e:70:15:9e:
10:59:be:76:e1:06:cd:a2:c3:ec:4f:70:da:5a:08:
ab:13:61:24:71:bb:d6:5f:01:73:c2:f5:8d:1d:21:
bf:d7:ed:55:ab:93:71:22:00:3d:12:2c:f7:2b:12:
34:cb:c2:b3:d1:06:0f:d7:be:e4:d5:1a:0c:75:59:
7f:58:1e:42:0e:a7:50:96:5a:02:ee:1d:5a:e5:5e:
f7:82:c1:8a:73:9d:df:99:5f:b2:e7:28:a5:81:dd:
b0:85:7c:d0:c5:33:03:a0:aa:7d:81:6e:e9:6d:ca:
8a:9b:a9:33:63:e7:04:19:c3:11:22:f9:12:62:12:
dc:36:8b:7b:01:71:e6:68:2f:4b:2e:8b:9d:dc:b3:
92:45
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Authority Key Identifier:
keyid:E8:05:25:55:99:F4:A1:22:62:E2:00:9F:00:BA:E6:A5:27:AB:8A:FC

X509v3 Subject Key Identifier:
F9:9B:C5:64:29:04:1E:F9:0B:BA:4B:97:28:9E:2B:84:AC:00:73:D5
X509v3 Basic Constraints:
CA:TRUE, pathlen:0
X509v3 Key Usage:
Certificate Sign
Signature Algorithm: sha256WithRSAEncryption
72:01:32:f5:1f:1b:7e:23:d4:0d:65:51:e1:43:f7:2a:ed:bd:
1d:86:e7:b0:79:7b:a8:04:a2:77:d6:40:b2:e8:f1:d1:6c:91:
8c:4b:30:48:6b:a1:8b:8e:12:8d:4e:e0:32:7d:4c:88:1d:20:
8c:74:8b:5a:0b:55:f4:e5:33:d9:ac:bf:fb:b3:7b:3a:52:41:
17:ab:7d:70:ec:42:73:39:06:3e:10:62:8e:f4:d4:36:ef:65:
5d:de:a2:15:f4:51:1d:19:26:a1:9b:00:46:86:67:b0:30:70:
33:d4:af:6f:cb:1b:63:92:b1:ec:0d:1f:bb:a8:9f:c1:c3:30:
f7:f9:83:70:c5:08:0b:9b:c9:2d:b7:0b:dd:e5:87:4a:d9:76:
84:6a:e5:34:95:f5:b9:22:73:d4:ec:b1:1f:98:1d:db:83:3d:
d5:e2:9c:59:86:66:bb:c5:f0:71:a4:07:31:58:f5:70:57:c6:
a2:91:04:5b:1b:07:94:0d:92:76:33:5f:f4:9a:18:ff:05:73:
c2:6e:fc:58:96:03:65:a0:a6:e5:78:a5:69:99:7a:36:9b:9f:
9a:df:d2:36:0a:80:d0:69:fe:68:f2:d3:10:f8:23:b3:38:b3:
be:5c:9f:c8:2d:4f:bb:cc:b5:4e:99:25:eb:3b:47:62:89:ab:
25:e8:1a:df

Certificate:
Data:
Version: 3 (0x2)
Serial Number: 1 (0x1)
Signature Algorithm: sha256WithRSAEncryption
Issuer: C=US, ST=California, L=San Diego, OU=General Use Test Key (for testing 13 only), OU=CDMA Technologies, O=QUALCOMM, CN=QCT Root CA 1
Validity
Not Before: Nov 22 06:26:53 2017 GMT
Not After : Nov 17 06:26:53 2037 GMT
Subject: C=US, ST=California, L=San Diego, OU=General Use Test Key (for testing 13 only), OU=CDMA Technologies, O=QUALCOMM, CN=QCT Root CA 1
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:c6:17:d6:98:f9:9a:39:e9:75:1a:79:55:66:07:
e8:42:1f:2d:ba:43:d7:fa:9a:e4:a1:9e:51:12:b8:
b6:fa:82:79:fc:0e:01:14:a5:6b:d4:67:68:d5:10:
99:70:d3:d7:6b:5c:61:23:08:28:9c:ae:ee:2e:4b:
c8:a2:d7:bf:6c:e4:5e:a3:9c:ff:01:8f:5c:a3:8a:
ad:3e:5c:6a:94:b1:67:cc:93:17:2d:c9:50:36:cc:
f2:59:d4:26:b3:b9:96:fc:e3:b5:92:a3:9d:c6:44:
f5:6d:42:e2:d1:27:c3:ab:07:4c:7c:e1:48:6e:88:
40:e2:7c:f0:c8:08:29:8b:b3:a4:ef:87:4f:13:42:
d0:34:16:8a:6e:f7:3d:ac:0a:ac:5d:3a:29:b4:ad:
06:36:53:ec:85:09:98:f8:0c:15:67:b1:e8:ba:5d:
46:f3:26:db:73:b5:b2:3f:84:5b:9c:3f:5f:ff:b9:
75:84:35:2a:69:f7:6e:89:67:af:7a:ae:cf:d6:37:
24:17:3f:38:d0:47:db:d3:a9:e0:f0:20:81:d3:2e:
ed:97:d5:dc:a3:98:c7:e1:3b:45:53:e1:a7:39:41:
63:3f:3c:e5:fc:84:13:15:47:00:f1:4b:54:bd:0c:
5b:20:5c:68:43:89:70:53:e4:cd:09:f6:f6:7f:50:
0a:53
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Subject Key Identifier:
E8:05:25:55:99:F4:A1:22:62:E2:00:9F:00:BA:E6:A5:27:AB:8A:FC
X509v3 Basic Constraints:
CA:TRUE
X509v3 Key Usage:
Certificate Sign, CRL Sign
Signature Algorithm: sha256WithRSAEncryption
0f:d5:72:da:d6:e5:f1:54:5f:3b:cd:d2:f3:67:ab:cd:11:3e:
0a:08:b0:f6:21:db:9a:42:43:cf:ae:fe:4c:dd:8d:83:9e:1d:
79:cb:79:46:42:fb:d8:06:11:a7:8b:1f:05:91:7f:72:65:a7:
cd:91:af:93:04:7d:f0:d6:87:55:67:1a:50:28:ea:89:6b:4f:
c1:3c:da:5c:25:45:2d:03:bb:b7:26:73:23:38:e0:c3:ec:85:
d2:c2:66:be:4d:1c:43:bd:2d:69:4f:c9:7a:51:1d:f8:31:53:
3b:b7:e8:de:62:33:5c:52:2f:50:b9:2a:e4:6d:61:42:27:f4:
22:af:ec:27:e3:94:9b:5a:0a:e0:5a:fe:97:89:06:04:bb:0f:
d8:55:af:1c:70:86:f6:61:12:ac:76:45:54:ef:9b:c8:4e:41:
cf:bc:f0:85:ce:d9:d2:99:5b:b9:1a:0a:93:ad:88:bd:54:70:
f7:c7:0f:14:7c:9e:e7:3d:3b:84:4a:3b:54:35:b6:03:a4:ee:
88:a7:17:2d:9d:ca:a1:a0:b6:99:83:66:b5:75:b3:ac:47:8d:
65:84:5a:07:8f:cf:c6:a8:e1:93:f2:3c:ac:63:eb:8d:1f:a2:
c3:4c:b8:4c:2a:e5:d9:51:59:73:8f:74:a4:87:42:e4:11:9d:
95:bc:7b:7c

So you may get lucky, not needing ratel signed edl/firehose programmer, but general/debug qualcomm only signed programmer could work.