Hi @adiueo
Here's a teardown [https://www.youtube.com/watch?v=9ky390kJxso|video] of the phone that may help to gain access so that you can replace the [https://www.deviceparts.com/volume-button-for-xiaomi-poco-m4-pro-5g-blue-ori.html|volume button] in the phone. The link is only to show what the part looks like. It's not a recommendation to use the supplier in the link.
Search online for ''Poco M4 Pro 5G volume button'' to find other suppliers that may suit you better.
Be careful as a lot, supply the power/volume button flex cable and not the button actuator as shown above. The actuator presses the switches on the flex cable in the phone.