Not sure. I just have an Umbrel. Not sure if REST is automatically installed or not.
Discussion
yeah i don't have a clue how umbrel works, check if you have RTL, REST should come as backend to it, also check btcsession on yt, maybe he has some tutorials
scroll to bottom section of this post https://community.getumbrel.com/t/how-to-use-tailscale-with-umbrel/6782