Why is my volume down button not working?

My iphone’s volume down button is not working.

It started to fail in december of last year. when I opened my iphone to find out, the button worked with the display off but not when the display was clipped into the housing. Then the button failed completely no matter whether the display was clipped on or not. I decided to ignore this and kept using my iphone without a functioning volume down button. A couple months later, I decided to finally fix this issue. I’ve tried replacing the volume/power ribbon cable and it still isn’t working. What’s wrong and what can I do about it?

Plus when I bought my iphone, it was in a rough condition and the housing was slightly bent along the the mute switch.

My iphone was also repaired by a third party technician for the audio IC(Loop Disease) issue.