Battery at 1%, searching for carrier and slow boot

Hi

A while ago I bought an iphone 7 (phone 1) that was used that had a broken screen. When I got it (phone 1) it wasn’t booting as it should, so I took out the battery and tried it in another iPhone 7 (phone 2). When I tried the battery in (phone 2), it (phone 2) wouldn’t boot normal either, and when it (phone 2) booted up the battery only says that it (phone 2) has 1% charge and when the lightning cable is plugged in it doesn’t say that it (phone 2) is charging. It isn’t recognized by iTunes neither. Also (phone 2) only stays at “searching“ on the top left corner (searching for carrier network). It (phone 2) is also really, really slow when in use. (Phone 2) worked as it should before I tested the battery from (phone 1). I have also confirmed that both of the batteries from (phone 1) and (phone 2) work as they should. (Phone 1) won’t recognize any audio device, so I suspect that it (phone 1) has a bad audio IC? But my question here is actually what is wrong with (phone 2) that worked before I tested the battery from (phone 1)? I am thinking that it might be the charging IC, but I am not sure.