6s Cellular Antenna Replacement or Motherboard Issue?

So I do multiple iPhone 6/6s repairs. I basically know the phone like the back of my hand at this point. I have a friend who bought an iPhone 6s, and was trying to activate it on Verizon. The phone is unlocked. My friend took it into the Verizon store, had a sim put into it, for it to not work quite right. I will list a video of it, that was screen recorded. It isn’t the common “no signal” motherboard problem I don’t believe. So, when you put the SIM card in the phone, it says no signal at first, then says “iPhone is not activated”, shows Verizon but no signal bars, then goes back to no signal. When it was taken into the Verizon store, the Verizon worker said that it was a “bad antenna”. The screen has a few dead pixels, meaning it probably is water damaged, so I am hoping it is a simple antenna issue and can be fixed by a simple replacement. I have a spare iPhone 6s so I can use any part of it to fix my friend’s. I just don’t want to open up the phone and take the time to try and repair something that is just a motherboard issue that I’m not able to repair. What do you think?

Heres the video- https://youtu.be/ZqLJrhTGia4