[Q] How to make vpn connection from behind a 3G router to a remote vpn server?

[Q] How to make vpn connection from behind a 3G router to a remote vpn server?

when i directly connect my pc to the usb 3G modem, the vpn works with little problem(time-out can only be solved by logout/login so far),
but if i connect my pc to a 3G router, i.e. my pc is now within the intranet behind the router, the vpn connection fails to be established...