bind x "exec \"configs\\dummy configs\\TripleflyON.cfg\""
%appdata%\Teeworlds\configs\dummy configs\TripleFlyON.cfg
sv_register 1
in autoexec_server.cfg or myServerconfig.cfg. Can google your router model and there might be a guide on how to port forward with it, it might also be hidden in some 'advanced' settings tab of the routers web GUI. I've never had a router that didn't allow it, but yeah I guess it's possible you simply can't on your router.