Why my phone's signal and wi-fi keep dropping and disconnecting? S6Edge

Hi I have Samsung galaxy s6 edge. It was not previously damaged or watered. Now I am getting signal problems on it. Sms are with delays, calls end soon after signal loses. Same problem with the wi-fi, when starting to use network on wi-fi it disconnects. It is android 6. I reinstalled android, did factory reset. Nothing helps. Helps only one way- when I start my phone with usb connected to CPU. Then everything works just great. I did not try yet to downgrade my android to lollipop? So could anyone help me with this serious issue. Maybe you had same problem? I hope it is not a mainboard crash...

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