Are you aware if your Gateway uses UPnP or not. Do you know how to log into its control centre?
At an estimation, the log in will be: type 192.168.0.1 in your browser address bar, leave username blank & admin for password. Else type 192.168.1.1 into your browser & both username & password admin. Check the options & look for UPnP. If there is none then you will need to forward a port. If there is UPnP & it is not enabled, then enable it & that should fix the issue. If there is UPnP & it is enabled, then perhaps it does not work efficiently & port forward is safest option.
If you need help port forwarding, let us know.