iPhone 6S showing No Service After Housing Replacement Not Blacklisted

Hi,

I would much appreciate if someone could help me with this one.

I do housing replacements for a long time now. I this time haven't tested the phone carefully before I made a replacement. It catches signal for 5 mins, then it suddenly starts showing no service for 4-5 secs, then the signal comes back on.

I replaced the charging port flex with a new one, then replaced the tiny ribbon that goes from the motherboard to the camera (at the top right), then cleaned with 99.9% Isopropanol the antenna bracket, checked the screws lengths on that side, on the top left corner, performed a restore and the problem is still there, goes to no signal every 5 mins.

What else I can do? I belive this is the hardware level problem, where the components that may cause the no signal issue? The motherboard looks undamaged. Please help.

Many thanks.