I don't use the 2310 firmware anymore, so can't remember the setting, but you will need to enable a few setting for a VPN to work. I use the Cisco VPN at work (vsn 1.5 or something)...
Now lets see (from memory)...
Make sure ALL the boxes are checked in the Advanced / Firewall / VPN Passthough screen.
I don't know if enabling the Gaming option would help, but it is supposed to help some things. Also, I don't know if you need to enable multicast streams... you could do worse than experiment and see if these two things help too! These two are under Advanced / Advanced Network ....
Good luck.