iPhone 6 signal error when lcd is back on

I have the same issue as many other guys in here, the signal problem... It says searching or no service, when the sim card is installed. This happend after a LCD replacement, and I have tried all different things. I heard of a guy who cleaned all the antenna cables + the sim reader. I did the same thing and connected my LCD, but without putting it into place I started it up. I unlocked my sim card, and it found service within 1-2 seconds, I was very happy. Then I turned off my iphone, and set the LCD into place. Ones it clicked into place I booted it up again, and now it's the same issue again.

I'm wondering why this happens? Mabye it is laying pressure on the cabels or the simreader?

- Looking forward to your answear! Thanks!