Problem iPhone, won't take service after numerous steps

Hi guys

To give you a full background on this, I usually buy faulty iPhones and repair them. 90% of my repairs so far has been successful. But this one, is something else.

At first, it was a bad deal, and I will never do that again. The screen was shattered, and the frame had a pretty bad bump at the bottom. Before I started the repair, everything was functional. After the screen replacement, I couldn't manage to activate the device, so I restored it. After restoring, it worked. But suddenly, the phone was just searching, or showing "no service". I tried all software solutions, but nothing worked.

I decided to replace the lightning dock, with the antenna. After replacing it, I got service, and full LTE. After one or two weeks I was going to sell it, but then I discovered that it had no service again. Bummer. I almost gave up, as I had fixed so many other problems with this phone as well.

But I decided to try and replace the frame, because of the bump at the bottom. I ordered a housing with parts pre mounted. After a while, I recived the frame, and installed the motherboard. I also gave it a new battery. Now the service worked great. I had LTE, and was really happy! I tested it many times. A week later, I powered it on and tested again. The service worked great.

So I felt safe it worked as it should, and then I sold it. After some days the guy who bought it contacts me and tells me there is no service. Bummer, again. He lives nearby, so I told him I would take it back.

I am very positive this is a motherboard issue. And I am very close on giving up. But if any of you have anything that might work, I would be very very grateful for it. I am not into micro soldering, and if I want to I have to ship it to UK (I live in Norway) and I have already invested way too much in it. Any ideas guys? Can it be the SIM reader?