Formgrader server connection refused

Apologies in advance for asking dumb question -

Set up:
Digital Ocean Droplet,
Ubuntu 22.04x64,
The Littlest Jupyterhub -
Jupyterhub 5.2.1
JupyterNotebook 7.3.3
Domain name set up and linked

The server appears to be running tljh which I can access via the domain name.
Problem: Server refuses to connect to formagrader - any ideas?