I noticed the phone I bought that it has a weak wifi and bluetooth signal after the deal. I tried opening and reconnecting...

Replacing the charging port should do the trick. I've repaired lots of iphones with this kind of issues and it leads to the charging port. Try cleaning first your headset port with a slim cotton swab and a little drop of alcohol on the swab. If it doesn't work replacing it will be your last resort.

Check your fuse and and relays.