I bought a used S8 and noticed signal was extremely $@$*!&, no 4g most of time, I opened and realized the mainboard and charging board didn't match, main was 950U and the charging board was 950F. I guess the guy broke the screen at some point and swapped his old board U but to an F screen/frame/charging board.
I replaced the charging board for a 950F so it matched main board and the problem is fixed. Btw je really careful with the broad connector between mainboard and charging board. The first replacement charging board F I screwed up bc i was trying to connect them kinda “blindly" and ended breaking connector, whereas the speaker would only work when I pushed the connection manually, had to buy a second charging board F and this time carefully connecting with mainboard gently and before puting the whole thing inside phone, so I could seeand handle the connectors correctly and not blinded