Ssh access to a server with internet access but no public IP address

I have an ssh server running Kubuntu 22.04 behind a router providing internet connection, but without a public IP (this is the policy of the ISP). So, the server can access the internet, but any port mapping between the router and the ssh server is useless, so the server is not directly accessible from the internet. I would like instead to be able to access the server via ssh from another internet host. If possible, I would like to do this without spending money. Is there a way to accomplish this? ---------- As correctly guessed in the comments, the ssh server is behind a CGNAT, so its router has no public IP address and is not reachable.