iPhone 6 with low / no gsm signal

Hi all, from one day to another my iphone 6 stopped to receive gsm signal, some times is low and on 3g somethimes it shows no service at all,

i've replaced the interconnection antenna and the charging flex where the gsm antenna is placed.

nothing happen.

is there any component on the motherboard that can affect that?

do you have any suggestions?

thank you