"No Service" after changing the back case

I had 2 iPhones 6. One with some motherboard water damage (let’s call it iPhone A) and one with broken LCD and bent case (iPhone B). I’ve ordered new screen and battery, took insides of the iPhone B and put them into case from iPhone A (along with new screen and battery). Everything seemed to work fine until i put the SIM in and there was “No Service”.

I was following this guide while switching cases. List of the little mistakes and notes from the process:

Step 13: I lost 3.1 mm screw. Should I try to replace it? Might it be the cause of this issue?

Step 31/32: I partially broke the plastic part of “angled logic board bracket “. I know that it holds metal against a golden dot. I tried to run the phone open and push metal to the golden dot with prying tool, but it didn’t seem to help the phone to find connection even If i restarted it.

Step 34: Somehow there was no “antenna interconnect cable “ in iPhone B, but I don’t recall having any network issues with it before switching the case… I put the one from the iPhone A in.

I skipped Steps 54 to 61.

Step 62: While putting speaker back in I broke plastic screw holder of the right bottom 2.3 mm screw. I noticed that this plastic part was holding some metal part below together with the case so i used 1.5 mm screw to at least connect this metal with the case.

I’ve read that popular cause for this issue is the lack of certain sticker on the antenna cable, but it seems to be in the proper place.

Also here are 2 photos of how the phone looks like inside If that’s any help:

I would be very grateful for all the tips on what might be causing this issue and how to fix it.