Resetting Ubuntu Core without messing up everything else

Hey guys,

sooooo I made a boo-boo while setting up a static IP address for my Nextcloud Box. Long story short, I cannot reach it via SSH anymore because it fails to get into my network. The next problem is: for some reason, it does not seem to provide me with any option to use keyboard and screen to create input on the actual device. Is it supposed to be like that? Even after restart, it just logs in automatically and then is on the screen telling me to use SSH (which does not work). So now I think the only possible way for me to get my nextcloud to work again is to reset the ubuntu installation. Unfortunately, I have quite a lot of data on my server at this point and I am a little worried that I am going to loose it (mostly because it is encrypted and I forgot to back up the decryption key before messing up the settings, my second boo boo). Is there a way for me to do that?

Thanks in advance for your help!

Oh dear. Backups are essential if you need to make this statement:

On the older pi image I could have linked to simple instructions to get you going again, but the newer Core is not so simple. Nor is doing so without the decryption key.

Do you recall how you set the network? Was it a subnet error or something easy to resolve like that?