So, I couldn't get umbrel to work, but I did get a nostr-rs-relay going using an Ubuntu install. It's messy, and I know there has to be an easier way, but proof of concept wise it's technically possible to run a relay on your phone.
Discussion
What did you use to make this work? Termux?
Yes, Andronix and Termux.
still messing with it a bit nostr-relay was easier to install. https://code.pobblelabs.org/fossil/nostr_relay/index
It lacks the nice status webpage that nostr-rs-relay has, but I couldn't actually get that to work.