The issue is not the name to IP translation, thus it is not No-IP that is the barrier. Neither is a requirement to have a static IP. The No-IP DUC discovers your public IP and sends that to its service to translate your name to that public IP. It can be a dynamic IP (which is the reason you need No-IP otherwise you could just have a static name to address in DNS).

The problem is how to get an inbound TCP connection from somewhere on the public internet into your PC running ACP so its web server can process that and return a response back out to the public internet. The fact that there are two hops through NAT networks creates the problem. There is nothing that can be done at the ACP end, it just listens on TCP/IP and responds to requests it gets.

The VPN idea is interesting except it would be a one to one service. Only one client on the public internet could talk to your ACP server. If that is good enough for you, that would be a solution. Set up the VPN between your computer (Mac, PC whatever), then edit the Hosts file to create the name to VPN address translation, and you could open your browser and go to the domain name for ACP. But no one else could be on that VPN unless they were on the client end of that same VPN.

You could set up a home net at the client end of the VPN so you could get to the IP but you would need to run your own DNS server to make that name available (or edit all of their Hosts file). It wouldn’t be awful to get there with ah IP address in the URL. And it would be a local IP so could be IPV4.

I m,,ay be wrong about there not being a way to get a TCP connection from the public net into your observatory computer. I also don’t think the cell companies are interested in providing server access.