LG G3 D855 dropping signal

Hi guys, my LG G3 started to drop network signal. When I switch to 3G only mode, it gets full tower signal, but suddenly after some time it drops to about 2 or 3 bars. Letting 4G mode I barely get 2 bars.

I tried 3 different ROMs and the issue continues. Is it something related to the antenna? Replacing it could resolve this?

Thanks!