They said it was fixed but within 4 hours the same problem is still there

Upgraded my contract in July.got myself a Xiaomi 12. Loved it. Perfect gift to myself for my birthday.last week noticed that the device just becomes hot without even being exposed to the sun or heat.took it in,guy tested it and within 5 minutes of him pressing it he confirmed that it does become hot.advised to book it in. Booked it in this past monday.no sms confirming that device was booked in. Today decided to pass by the shop. Was told that my device is fixed &ready.collected it.got home and there's load shedding.used the device's torch to get some light as I was entering my house. Got inside and went straight to the room to look for the lamp.torch goes off on my phone. When I unlock it the message "your device is too hot" appeared. I would like to have a word with the technician that "fixed" my phone.he has to tell me what exactly did he fix as the phone has the same problem I took it in for.also this device is insured and still under warranty as it is less than 6 months old.MTN please advise me as to what to do now since your technician failed to fix the problem but instead decided to return the device and I was told it's fixed.rather cancel the contract and take your device and give me my money back.clearly you don't want me as your customer