How I fixed this without sending it to have audio ic done I remembered iPhone 4 had a problem with audio chip and you had to put something on top of chip and put the lid back on to hold the chip down which worked a charm all I did on iPhone 7 was take logic board out locate the chip but don’t take the black tape that covers it off just rub it ware the chip is so you get the outline in the black tape then I used the card from cigarettes papers I cut a strip exactly the same size then folded it 3 or 4 times the same size as the chip I used double sided tape to stick it on top of the outline on the black tape put the board back in put all the standoff screws back in attach screen a test before fully putting back together my phone is still working well with just that bit of pressure

I've fixed this problem on a lot of iPhone 6 and it's always same thing a simple test is to place your phone screen down on a flat surface like a table then push down on each of the corners if the phone rocks your case is bent remove screen and straighten the case until it does not rock and is not twisted put back together and your signal problems will be gone