It's possible, but not sure if it'll require some code. https://github.com/aljazceru/awesome-nostr#relays is a list of various relay software. If you have experience running VMs or containers on mobile then these should all work at least minimally through such a setup.

Some concerns on mobile are that relays can use a lot of bandwidth and should be always online and routable from the public internet for general use. It doesn't make sense to publish to a relay if your intended audience can't get to it.