[Xiaomi Mi 8 Questions & Answers] Gopena:
So, I was having this issue about a week ago, and I think I managed to get it figured out- keep in mind this is all with the mi-globe "flavor" of xiaomi.eu, which itself is based off china version(s). I have a china Mi 8- I am in USA - I am on Verizon (CDMA/LTE)

Even if it's not necessary- I still do a complete wipe, flash china vendor, china system, then rom.

Anyway, I was having network & APN issues on current beta- 9.7.18. very similar to how you have described, APN's do not auto-populate, can't connect to data, can't make calls, etc. Tried re-flash and would sometimes be fine after a boot, but would then lose signal and not reconnect. I could swap the sim into the opposite tray and it would work again until reboot. I also had issues with WiFi and DNS (Kept getting stuck at "getting IP address" in Wifi connect)

I actually have yet to try doing a miFlash to "get back to normal/zero/baseline"- last time I tried I was having device version issues (phone not being recognized as a "dipper" in fastboot (?))

But regardless- this is what's gotten everything working again consistently. - TO CURRENT Mi-Globe/xiaomi.eu STABLE 10.3.4.0

I'm also using the "official" twrp downloaded from the twrp site (i.e. the one that does not have the notch enabled, not the one from the xda thread.) It works for me but may not work for you.

BEFORE YOU WIPE (I try to do this every time- not sure if it helps)

Sign out of your mi account - doesn't matter if you keep or delete but sign out and make sure you get the bug that find my device is off, too.

Sign out of your google account

Turn off PIN/FP/Pattern Lock - Turn off any lockscreen lock.

Full wipe (all the check-boxes AND the separate option where you type "yes")

Reboot Recovery

Copy over China 10.3.4.0 Vendor Image (extract from fastboot/miflash update)

Copy over China 10.3.4.0 Firmware Flashable .zip (From xiaomifirmwareupdater dot com)

Copy over Mi Globe 10.3.4.0 - With cache clear option checked, and magisk [see note at end]

Flash vendor.img

Flash firmware zip

Flash Mi Globe/xiaomi.eu

reboot.

do the setup

once you're at the home screen

Settings> SIM Cards & Mobile Networks> Select your SIM (First option(s))

Make sure Preferred Network Type matches your service (Mine is set to "Prefer LTE")

You should be good here. if not, try these:

Settings> SIM Cards & Mobile Networks> Select your SIM (First option(s))> Mobile Networks

Turn off Automatically Select Network. Wait a minute or five for the list to populate.

Select your network. If there are multiple options, select the one with the proper service for your plan (i.e if it's "YOURNETWORK 4G" "YOURNETWORK 3G" Select the 4G one if you're supposed to have 4G)

OR/AND

Settings> SIM Cards & Mobile Networks> Data Roaming

Experiment with this setting. Mine is set to "National Roaming" ON but "Overseas Data Roaming" NEVER.

ALSO

I recommend this one for everyone- one of my biggest issues was resolved with doing this EVERY TIME in MIUI

BYPASS VoLTE CHECK - this fixed weird data issues, and enabled me to use mobile data WHILE ON A CALL without WiFi- I didn't realize it was this option which caused that behavior, I kept thinking I flashed wrong or there was a bug.

Open Dialer and type: *#*#86583#*#*

You'll get a bug saying something to the effect of "Carrier VoLTE check disabled" and it (should) automatically enable.

To Check: Settings> SIM Cards & Mobile Networks> Select SIM- and you should see, bottom option on next screen, "VoLTE Enabled" and turned on.

RANDOM APN ISSUES

Make sure on your list of APN's that you're getting ones with these types of "APN type": "default" "supl" "mms"

it may be across a bunch of different APN's or all on one. Mine is all on one, so in the apn settings (Settings> SIM Cards & Mobile Networks> Select SIM> Access Point Names> Tap gray ">" next to each one to check/edit/delete) the field for my main APN's type, "APN type" reads "default,supl,mms" (No spaces)

Additionally

I really think these provisioning issues come down to differences arising from using a device on the network that not really expected to be used. Networks like mine (verizon) tend to do some funky tech implementations, namely, in my case, of VoLTE. Verizon supports it fully, and is fully built-out for it.

But it's not that simple!

Verizon (and other networks) select certain devices that will get to use this functionality and do not make it available to every phone they sell, so when I hook up an Mi 8, I have to tell the phone to do VoLTE even if it thinks it cant, because Verizon is not expecting a Mi 8 on their network.