MKR1400 signal problem

Hello, I have a really strange problem with my Arduino MKR 1400 board.
Since the very beginning, the board got stuck on the GSM.begin() function. So I bought several different antennas but the result is equal. The board refuses to connect to network (+CREG: 0,0) but after uploading several different codes (NetworkScanner example and others) the board connects and work fine for approximately 24 hours. Then the problem occurs again.
Right now uploading different sketches is not solving the problem. I think it must be modem fault, but it is really confusing.

When the problem occurs the board is having some signal problem (+CSQ:99,99), but another time, in the same place and same configuration the board was working fine.

Any help would be very appreciated.

Hi @kalutep

I would suggest you try the suggestion on this post: