Second hand phones

I purchased the Honor X9b 5G through an MTN contract four months ago. Shortly afterward, I started experiencing issues with the phone, where it would glitch during certain calls and then restart on its own. I took it to the Vodacom main store at Canal Walk five weeks ago to have it inspected and repaired. While they indicated it had been fixed, the glitching resumed within a few days, and the issue has since worsened.

It has become so problematic that the phone restarts whether I'm actively using it or receiving calls, and during these glitches, the screen turns off, preventing me from answering calls. I'm concerned about the quality and condition of the device, as this problem persists. Could someone please assist with resolving this issue?