Help with connecting to VPN through an ad-hoc network?

Strange set up - 4G USB dongle into laptop - laptop creates ad-hoc network with ethernet out - ethernet out goes into linksys wireless router - wireless router creates wireless network which various devices can connect to (ipad, PS3, etc.). There is a reason for this crazy set up - Saudi Arabia is not great for communications.

However I now have a problem - I cannot access my VPN so I can watch BBC iplayer etc on my ipad - I just get the message "You were disconnected by the communication device".

The Linksys has had VPN and PPTP enabled so it is not this I don't think - I think it is something on my laptop - can anyone give me a clue as to what this might be??

Thanks

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).