Lock button on iPhone 8 is broke. Who can fix?

Lock button on iPhone 8 is broke. Who can fix? I traded in AT&T one month ago, 8+ for 12Max Pro. Only gonna be extra 12 a month til new phones paid off. I reset the 8+ and turned find my iPhone off then gave my older brother the phone to turn into AT&T. Somehow in his back pack the 8+ was knocked around and it’s asking foe a 4digit pin to unlock. The absolute kicker is that the lock button on the 8+ doesn’t work at all so I can’t put the phone in recovery mode or anything. I always kept a good case on the 8+ so I don’t know why or how to fix. Please help