Extreme signal fluctuation sim reading again and again

Hello

I have this weird problem in my iphone 6

Lately it started todo extreme signal fluctations. The signals goes from full to two then to one and then to no service in the matter of seconds. And then it goes back full strength in the same time of window.

Also i have noticed that when i plug out the sim card. The phone continues to say no sim card installed again and again in the same matter of time. Seems like the phone is reading the sim again and again and making my phone to search for the reception again and again.

Performed things

Rebooted mutliple times.

New firmware installed

Updated network settings.

Resetted the network settings.

Changed the lower charging antina strip.

Changed the antinna on the upper left corner of the logic board.

Tried other network sims.