What is my free server address, and does it ever change?

Every server gets a free subdomain automatically, in the form <your-server-id>.play.zerosrp.co.uk. It never changes for the lifetime of your server and points at the same underlying IP as your server's raw address, so there is no performance difference between using the subdomain and the raw IP.

For Minecraft specifically, a DNS SRV record is created alongside the subdomain, so players can connect using just the hostname with no port number needed.

You can find your server's address on the Server Vitals page. If it ever fails to provision (rare), you will see a warning banner on the Vitals or Settings page with a link to contact support; include your server's ID when you reach out.