Sorry you’re going through that OP. I manage around a dozen large business Synology servers, and this right here is why I cringe when I see it be so often recommended for home use. I appreciate the value they bring for home labs with all their out of the box bells and whistles, but when they fail, they fail really hard.
For your use case, your best bet is to ssh in and start digging through the logs. You’ll be missing a lot of basic tools when you ssh in, like a text editor, and there is probably no package manager to install anything from the shell. I remember last time I had to do that I had to install some stuff by downloading directly. I believe I was also able to mount a remote drive from the shell and copy the logs over at one point.
Anyway, the log files should at least give you plenty of error messages and at least point you in the right direction for working with support. Good luck.