For the $5 first payment, they have a partnership with Snikket who will setup and administer a private XMPP server for you. It’s included in the price if service and is retained for as long as you maintain a server. You can use it for any XMPP thing you want, and have as many users on it as you want.
FYI, this is called “split horizon” DNS, where the location you get directed is based on the network you adk from. Make sure you have short TTLs set on your DNS records, or devices can have problems moving between networks and still having records cached from the prior network