edit: ok, i had the exact same problem stemming from the same sequence of events, and now i got my lte back. there's no weird process to do it, i just re-flashed the stock rom (without erasing modems) and did what it said in this thread:
https://forum.xda-developers.com/g5-plus/how-to/fix-persist-resolve-imei0-explanation-t3825147
so basically, steps are:
1. through adb, flash stock rom
2. through adb, flash twrp
3. through twrp, flash magisk
4. download busybox and a terminal emulator from the play store
5. install busybox to /system/xbin through the app
6. open the terminal emulator and type these commands:
$ su
# chown -R rfs:rfs /persist/rfs
# chown -R rfs:rfs_shared /persist/hlos_rfs
7. reboot
personally, if i flash another different rom i'll definitely be backing up EVERYTHING beforehand.