Poor cellular connection after replacing charging port

The charge port on these also contains a lot of the components for cellular connectivity. And swapping with a different board could definitely affect the cellular connectivity because the same bands and signal types are not necessarily supported. Your best bet would be to know what board it originally had, but as far as I can tell there are only 3 types for the S20 and you've already tried the one that is supposedly for the model you have. (As far as I can tell, it should be the "B" type) 980F is the international variant and DS for Dual SIM.

If you're still getting poor signal I would check to make sure that the thinner cable that goes from the daughter board to the main board is connected correctly and that connectors on both ends are clean and free of damage. It may also be worth it to try a different B board because the first one could have been defective.

Make sure all the little clips on both sides of the board are intact and in good shape. It's easy to bend them, and they are all over the main board and the charging board. Something could have been bent or broken during removal of the back glass. Often they are ground or antenna connections necessary for good signal.

Otherwise, I would make sure nothing on the main board was accidentally nudged when you did the original repair. Especially components in the area of the cables between the two boards.