iPhone 12 pro max no WiFi, Bluetooth, or cellular data

Wow, talk about a mess of problems!

Seriously, although it sounds like you're having all sorts of issues, IMHO you've really only got two; the reboots and your screen. Let's address the screen first.

We've been seeing this problem more frequently on the later model iPhones, the ones after the iPhone X series. What's happened to you is that the mounting frame that's supposed to be securely glued to the display has separated from the screen and stayed on the phone instead of coming off with the display assembly. Take a look at the back of a screen assembly and compare that to your phone.

See that metal frame all around the edge with the big chunk of metal (the shield you've been puzzling about)? You'll find it's missing from your screen and instead is still on the phone's midframe. The giveaway is the fact that your front camera assembly is still inside the phone rather than on the display as it should be.

The cure for this is to go around the edges of the phone with your opening picks, only this time get under that frame. Just repeat the whole opening procedure on the frame, but be careful about bending the frame since it doesn't have the display as backing now. Since the OLED is off, you can also slide a tool under it from the inside to help pry it up, being careful not to damage any parts or the battery.

Once you have it off, you'll need to buy some glue (look for something like B-7000, along those lines) so you can resecure the screen's frame back on the screen. I've had this happen to me on some very cheap aftermarket screens on an iPhone X, and honestly didn't have any luck; when it separated from the screen frame it damaged the OLED. Hopefully yours turns out better.

Okay so there's the screen, now lets look at the reboots. These sound like the phenomenon known as a kernel panic. The kernel is the heart of the iOS operating system, basically running the show. One of its functions is to scan all the phone's sensors every three minutes. If it finds a sensor it can't read, it records the problem and its cause in a file called a panic log then reboots the phone in an attempt to recover from an unrecoverable error.

Unfortunately, since this is a hardware error it's still going to be there the next time the phone goes to scan its sensors. Since computers can only do what they're told, it will simply keep repeating that scenario to infinity and beyond.

Luckily we have a wiki article all about these kernel panics written by a former Apple repair technician that is invaluable in figuring out these problems.

iPhone Kernel Panics - iFixit

Your homework assignment is to read the section titled "How to find a panic log", then use that information to locate the latest panic log on your phone. Take a screenshot or copy & paste the first page of the log then come back here and add that information to your question. Once we have that information we'll be able to diagnose the actual reason for all your restarts and hopefully tell you what part needs to be replaced in order to put those panics to rest once and for all.