You can use dynamic DNS if the IP address you're getting is a public one.
There are also services which you can get a reserved IP with all ports open, and then you wire guard tunnel it to your server.
Reverse proxy with a dynamic DNS server (DuckDNS for example)
A recent post on HN mentioned use of a Cloudfare tunnel. Apparently that allows for self-hosting behind a router with no incoming ports needing to be opened.
Give them a call? Or switch ISP if yours doesn't allow it. Otherwise you need a relay service, at which point you might as well host with them directly.
I'm in a weird situation, hence my confusion. I live next door to an Xfinity hotspot. I signed up with Xfinity so they sent me a modem. I couldn't get it to work, but I found I could just use the hotspot. It works great but it serves potentially thousands of users so it's not personal to me.
That's an interesting situation indeed! Hosting a service on there might be rather tricky but if you do get support to punch through the CGNAT on it (I presume they use that) it will be even funnier :D