@jeffin since none of those buttons work, replace the flex cable. Use something like [https://www.youtube.com/watch?v=xSgR9DXvgY8|this video] to get to it. For the flex cable itself, [https://www.nadiemellamagallina.com/en/manuales/smartphones-1/xiaomi-191/xiaomi-mi-9t-1121/pulsadores-laterales-9868|this guide] will work. It was written in Spanish; use a browser that does the translating for you (if needed).
Th flex cable is readily available at many online stores. You are looking for something [https://www.amazon.com/AYSSJ-Power-Button-xiaomi-CRNKJ10889/dp/B0CX5M8KY5?gQT=2|like this] Example only and not an endorsement.