It seems that the problem is more related to the outgoing traffic rather than the incoming. At the remote location (my workplace), we have 2 routers in series. The one closest to the internet is a Draytek Vigor, and the one behind that is a Belkin. The Draytek provides Guest Internet without them being able to access the company network. The Belkin acts simply as a firewall (inbound) and NAT device and seperates the guest accessible netwrk from the company network. On both routers, outgoing services are unrestricted (allow all)
When I tried this morning, I was connected on the Belkin (inside the company). No connection. If I reconnect to the Draytek (Guest internet), it works. The router at home is a Netgear, and it seems to be OK for incoming connections, but not for outgoing. The Belkin also doesnt seem to be happy with outgoing connections. The Draytek Vigor however is. It seems like the outgoing connection is quite "fussy", and I am hoping that I do not run into the same issue if using other outgoing internet connections - For example, in a hotel or via 3G mobile service - I can readily control the conditions of my home internet connection, but not that of out and about connections.
With the radio RRC being in the DMZ back home, I can readily navigate to its web server set up page from both work networks. So there is no issue with the TCP/IP connection to Port 80. It just seems to be the SIP connection to port 13000 that is fussy.
So, I have it worked around, for test purposes at least. The concern is that if I am somewhere that has as fussy an outward connection, then I will not be able to connect again. I guess all I can do is try and see what happens!