If the console is clean from inside as well as the ventilation vents, and the thermal past/pads are replaced/in good condition, and the cooling fan is running properly, Then its mostly the heat sensor which is built inside the APU, Which is unfortunately unrepairable without replacing the APU If its available on aliexpress or ebay, or the motherboard itself, which doesn't make sense and i consider it beyond economical repair, your better off buy another console.
I'm sorry for the unpleasant experience you've had, but the shop you've bout it from has cheated you, reliability on the other hand is something that has never been associated with Microsoft consoles too.
hopefully you can get another one soon.