How would I go about fixing SIM card not registering after flashing a custom ROM made for a clone of my device
I fixed it by doing the following:
Wiping Dalvik/ART Cache, Data, System, and Cache.
Flashing the ROM (Resurrection Remix).
Flashing the kernel fix.
Restoring Tee, NVData, and NVRam from my TWRP backup.

See this XDA thread for extra details.