Hi Martin,
sorry for my bad english, just imagine my german accent while reading and have fun with it.
So i had the same problem with a iPhone6. After trying almost everthing i could read here,
i found the "connection sticker" on the clear copper field of the double antenna flex. https://d3nevzfk7ii3be.cloudfront.net/igi/1j2pnFoGLBbwfkED.standard
This sticker was not on the copper and coudn't connect to the metal peace behind the display.
I put the sticker back in its position and the phone back together. After a restart and a reset
of the network settings the iPhone quickly found a Service.
I hope this helps!
Stefan