A connection is never to slow for the SIP connection, but if it 's to slow you will hear it when connecting the audio. I guess it's the router at the radio site which fails, Try first to assign the RRC as DMZ- server or exposed host or what ever its called in your router. If you can connect remotely to the webinterface of the Radio-RRC it's probably something with how it handles port 5060, try to change the port number. Also check that there is nothing called SIP ALG activated. Also try to upgrade the router firmware, there are lot of bad firmwares in the routers. Branded routers provided by the ISP is worst, they may never handle portforwarding in a proper way.
73 de mike