I only have a modem connection to the outside facing NIC, the routers are connected to the internal NIC, which I can get (or at least have got) VPN connection on using one of the settings suggested. Office issue I can understand, I have similar problem with ssh to my server from the office, as they block outgoing traffic on port 22, so I have to port forward from another port number back to 22 within the LMCE firewall. however, I have been trying using my Android mobile mainly, or, my laptop pointing at the external DNS entry. Both of which I can get to work from internal to the network, using 192.168.80.1 as the host.
Not sure if it is the modem how I can test / verify that?! Any ideas?