iPhone 6 reads Sims card but doesnt work after "custom repair"

Model: iPhone 6 Space Grey

iOS: 10.2

Bought a grade C iPhone 6 from eBay

(Made calls, flashlight worked, charged but mic doesn't work, overall worked)

Bought a blue housing for iPhone 6 for fun

Took it in to get switched

Has problems

(Note: Yes I know what I was doing. Taking a crashed Ferrari to work with Toyota Camry parts but im here just to ask about one issue.)

Problem I like to address when the iPhone reads Sim card.

Goes from on to off, and sometimes never works. Goes from 4G to LTE, then LTE stops working and reads as 4G. When LTE is back and I go to vocemail it switched to 4G and fails and thats when calls fail too. I hope this can be fixed, if not ill resume using this as a "iPod". If there is, any link to parts will help.! <3

I have tried resetting network settings, factory default and carrier update.

Other note: Is using a non-apple battery, rip performance. Noting if related to Sims error.