Connecting to the external Domain name for a local computer would not actually take you past or even to your ISP (excluding DNS lookup). What happens is the client computer does the DNS lookup and gets the IP address for the external interface of your router. When it connects to this IP, it's connecting directly to your router, and your router is forwarding that connection back to the computer addressed in the virtual server setting. The connection wouldn't be blocked by any existing firewalls or restrictions set by your ISP, because it never goes through their servers.
If you have the capability, try setting up an SSH server, and set up your telnet session through that. As insecure as telnet is, I don't think I would want it open to anyone to access. As an added bonus though, SSH should increase the reliability of the telnet session.