I never tried using it that way. I use localhost only.
Some people configured it to use a .onion service using orbot.
Discussion
My ISP doesn't give access to the router so my only option if I wanted to make a public relay on the phone would be getting a vps and setting up wireguard
Don't know if there's another way
If you have access to your router I think you can setup some port forwarding for this but I don't know much about networking
If you don't mind your local relay can be accessed publicly by others then you can probably try reverse tunnel using reverse tunnel service such as
https://github.com/cloudflare/cloudflared
Run command in termux:
cloudflared tunnel --url http://localhost:port
It will give free accesible random domain to access our local relay.