I've been having a devilishly hard time trying to get VPN to work with my DGL-4300. I finally got it to work (and am posting through the tunnel) by changing the following router configuration options:
Advanced -> Firewall -> (ALG options) IPSec UNTICK THAT BOX
I read an old thread here that suggested unticking the PPTP box - that didn't help me, but it might help others if unchecking IPSec doesn't.
