RDP in VPN Tunnel Fails When Connected to Mobile Broadband

I have a number of remote users who are issued mobile broadband devices from one of two mobile broadband providers.

These users connect to mobile broadband and then create a VPN connection to the main office and then initiate an RDP connection to a server located in the main office.

A handful of remote users who try to connect to the main office using one mobile broadband provider are able to create the VPN but when they try to connect to the RDP server the connection times out.

I changed the client RDP settings for low bandwidth and the RDP connection still fails.

Does anyone have any ideas on what is causing the RDP connection to fail even though the VPN connection succeeds? It seems like remote users who have this problem are only users with one mobile broadband provider while mobile broadband users of the other provider are able to connect to the VPN and RDP server.

Edit: I could be wrong in that it may be users of both mobile broadband providers have problems connecting to the RDP server after they are logged into the VPN. I have only witnessed the error happen with users of one of the mobile broadband providers though. I can say that users connected remotely not using the mobile broadband devices do not have any problems connecting to the VPN or RDP server.