Yeah, that's the problem with back glass replacements - you end up exposing the phone's internal components to the high heat needed to get the glass off. In addition, since the wireless charging coil is exposed, it's susceptible to a random piece of glass or a slip of the prying tool.
On the iPhone 12 Pro Max, the power button is integrated with the volume buttons, the ring/silent switch and the wireless charging coil. Chances are very good either the heat or a tool damaged that assembly, resulting in a non-functional power button. Here's the part you need.
iPhone 12 Pro Max Audio Control Cable and Wireless Charging Antenna
Unfortunately I haven't been able to find any guides to replacing that part. The best I can suggest is to watch some teardown videos and use what existing iFixit guides there are to take out the components as they're encountered in the videos. Here are a couple to get you started.
iPhone 12 Pro Max Teardown - iFixit
iPhone 12 Pro Max Teardown - YouTube
Since you're blazing new trails here, it would be awesome if you could take pictures along the way and create a guide for others who end up with the same problem.