Why is my replacement screen not working? OnePlus Nord

Hello! I broke the screen and LCD of my OnePlus Nord (AC2003 Model). Everything else seems to be working (my alarm went off as planned), but the battery eventually died. I've bought a replacement screen, and after removing the motherboard and daughterboard (following the guide on here), I'm connecting my new screen with the display connector. (I want to test that it is working before finishing installing it.)

The screen comes alive, lights up with the Onplus logo and starts loading, but then after a few seconds, it freezes and goes black, sometimes with a buzz. Once or twice, I got to the pocket-mode home screen, but I can never keep it alive for longer. Plugging it in to a charger doesn't seem to be helping. Sometimes it won't come on at all. Disconnecting and reconnecting the battery connector seems to be helping, but not for long. Then it'll begin restarting itself over and over unprompted.

What is the problem? Could the battery be fried/damaged? Does it mean there's some more serious problem with the phone hardware? Is it an issue with the new screen? It feels like it keeps starting and then short-circuiting, but I don't really know what I am doing. Help!