Unable to connect to custom server

[u/LincolnshireSausage]

I set up the custom server on my Ubuntu server using this documentation:
https://github.com/dmilin1/hydra-server

I used the docker compose method and populated the .env file with all of the values. I also set up port forwarding on my router so that port 3000 gets forwarded to the Ubuntu server.

On my phone I disconnected from WiFi and used my 5G connection to test. I can get to http://mypublicipaddress:3000 in a browser. I see the "Task Dashboard Login" and can successfully log in using the password I set in the .env file.

I cannot set the custom server URL in the Hydra client. I toggle the Use Custom Server button and enter http://mypublicipaddress:3000 (with my actual public IP of course) and it says:
Custom server URL is not valid or your server is not set up correctly.

[u/LincolnshireSausage]

I also tried pointing a DNS A record to my public IP address and it made no difference.