You can use the NostrOS client on Android. Connect your umbrel relay and it has a "push to relay" option to send all your previous notes to a relay

https://github.com/KoalaSat/nostros/releases

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Tried to do that just now, though got stuck at not being able to add my umbrel relay to it, giving me a bad format error unfortunately.

Try to connect via Tailscale app in umbrel using a ip number.