Even in this situation, your ISP can port forward to you.
While not universal, some ISPs support PCP, where you can ask for a port mapping to your CGNAT-ed IP and port. They might or might not honor the external port (if it is taken, they obviously cannot), but you will get some hole punched.
Some do. But when they don't, it is not a fault of CGNAT - which does provide the capability -- but a fault of specific ISP, that's not willing to use it.
While not universal, some ISPs support PCP, where you can ask for a port mapping to your CGNAT-ed IP and port. They might or might not honor the external port (if it is taken, they obviously cannot), but you will get some hole punched.