Network signal comes and goes, even after restore?

I have this iPhone 6S, the network comes, works, but once in a while, signal suddenly drops from full to 1 bar then No service, before coming back to full. Essentially, it kind of resets.

I have tried restoring it multiple times, even taking the logic board to a different phone/casing which led me to believe that the problem was in the logic board.

Any ideas?