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.