[ZTE Axon 7 ROMs, Kernels, Recoveries, & Other Deve] manu7irl:
From our system tree:

~ # ls -al /dev/block/bootdevice/by-name/
lrwxrwxrwx 1 root root 16 Jan 4 07:31 aboot -> /dev/block/sde11
lrwxrwxrwx 1 root root 16 Jan 4 07:31 apdp -> /dev/block/sde21
lrwxrwxrwx 1 root root 16 Jan 4 07:31 bluetooth -> /dev/block/sde16
lrwxrwxrwx 1 root root 16 Jan 4 07:31 boot -> /dev/block/sde12
lrwxrwxrwx 1 root root 15 Jan 4 07:31 cache -> /dev/block/sda2
lrwxrwxrwx 1 root root 15 Jan 4 07:31 cdt -> /dev/block/sdd2
lrwxrwxrwx 1 root root 16 Jan 4 07:31 cmnlib -> /dev/block/sde19
lrwxrwxrwx 1 root root 16 Jan 4 07:31 cmnlib64 -> /dev/block/sde20
lrwxrwxrwx 1 root root 15 Jan 4 07:31 cryptkey -> /dev/block/sda8
lrwxrwxrwx 1 root root 15 Jan 4 07:31 ddr -> /dev/block/sdd3
lrwxrwxrwx 1 root root 15 Jan 4 07:31 devcfg -> /dev/block/sda5
lrwxrwxrwx 1 root root 16 Jan 4 07:31 devinfo -> /dev/block/sde15
lrwxrwxrwx 1 root root 15 Jan 4 07:31 dip -> /dev/block/sde9
lrwxrwxrwx 1 root root 16 Jan 4 07:31 dpo -> /dev/block/sde23
lrwxrwxrwx 1 root root 15 Jan 4 07:31 dsp -> /dev/block/sde8
lrwxrwxrwx 1 root root 16 Jan 4 07:31 echarge -> /dev/block/sde25
lrwxrwxrwx 1 root root 15 Jan 4 07:31 fbop -> /dev/block/sda6
lrwxrwxrwx 1 root root 15 Jan 4 07:31 frp -> /dev/block/sda7
lrwxrwxrwx 1 root root 15 Jan 4 07:31 fsc -> /dev/block/sdf3
lrwxrwxrwx 1 root root 15 Jan 4 07:31 fsg -> /dev/block/sde4
lrwxrwxrwx 1 root root 15 Jan 4 07:31 hyp -> /dev/block/sde3
lrwxrwxrwx 1 root root 16 Jan 4 07:31 keymaster -> /dev/block/sde18
lrwxrwxrwx 1 root root 15 Jan 4 07:31 keystore -> /dev/block/sda4
lrwxrwxrwx 1 root root 16 Jan 4 07:31 lksecapp -> /dev/block/sde17
lrwxrwxrwx 1 root root 16 Jan 4 07:31 mdtp -> /dev/block/sde10
lrwxrwxrwx 1 root root 15 Jan 4 07:31 misc -> /dev/block/sda3
lrwxrwxrwx 1 root root 15 Jan 4 07:31 modem -> /dev/block/sde7
lrwxrwxrwx 1 root root 15 Jan 4 07:31 modemst1 -> /dev/block/sdf1
lrwxrwxrwx 1 root root 15 Jan 4 07:31 modemst2 -> /dev/block/sdf2
lrwxrwxrwx 1 root root 16 Jan 4 07:31 msadp -> /dev/block/sde22
lrwxrwxrwx 1 root root 15 Jan 4 07:31 persist -> /dev/block/sdf5
lrwxrwxrwx 1 root root 15 Jan 4 07:31 pmic -> /dev/block/sde6
lrwxrwxrwx 1 root root 16 Jan 4 07:31 recovery -> /dev/block/sde14
lrwxrwxrwx 1 root root 15 Jan 4 07:31 reserve -> /dev/block/sdd1
lrwxrwxrwx 1 root root 15 Jan 4 07:31 rpm -> /dev/block/sde1
lrwxrwxrwx 1 root root 15 Jan 4 07:31 sec -> /dev/block/sde5
lrwxrwxrwx 1 root root 16 Jan 4 07:31 splash -> /dev/block/sde24
lrwxrwxrwx 1 root root 15 Jan 4 07:31 ssd -> /dev/block/sda1
lrwxrwxrwx 1 root root 16 Jan 4 07:31 sti -> /dev/block/sde26
lrwxrwxrwx 1 root root 16 Jan 4 07:31 system -> /dev/block/sde13
lrwxrwxrwx 1 root root 15 Jan 4 07:31 tz -> /dev/block/sde2
lrwxrwxrwx 1 root root 15 Jan 4 07:31 userdata -> /dev/block/sda9
lrwxrwxrwx 1 root root 15 Jan 4 07:31 xbl -> /dev/block/sdb1
lrwxrwxrwx 1 root root 15 Jan 4 07:31 xblbak -> /dev/block/sdc1
lrwxrwxrwx 1 root root 15 Jan 4 07:31 ztecfg -> /dev/block/sdf4

My bad,

the commands will be:

adb shell dd if=/dev/zero of=/dev/block/bootdevice/by-name/modemst1 bs=16384

adb shell dd if=/dev/zero of=/dev/block/bootdevice/by-name/modemst2 bs=16384

I have another problem, my device is still asking for default password in TWRP,

I tried to first install the rooted boot for B20 then WIPE DATA and CACHE from Stock recovery, then flashed TWRP, then reboot after the initial setup, I rebooted into TWRP to see the password prompt...

The no-verity update zip just bricks my device and I forced me to reflash the modified boot and TWRP... Then still the same password prompt showed up...

---------- Post added at 10:19 PM ---------- Previous post was at 10:06 PM ----------

This command was incomplete

adb shell dd if=/dev/block/bootdevice/by-name/modem of=/sdcard/modem.img

you could also pull directly to the computer via adb pull...