I know this thread is a few months old, but I was pulling my hair out for over a year trying to figure out why my VPN kept disconnecting after 3 minutes whenever I was connected via router, but worked fine when I connected directly to my DSL modem.
I searched everywhere on the internet, and couldn't find a solution to this problem. So, hopefully, this will help others searching online trying to figure out how to solve this particular VPN disconnect issue.
Anyway, the solution lies within how the modem and router are configured for PPPoE and NAT. I'm not going to try and explain what that stuff means (maybe someone else can), because I don't understand it. But, long story short, I logged into my modem (in my case, my DSL modem had an address of 192.168.something) and noticed that it was set for PPPoE on the modem level. What I had to do was change that setting so that it was performed at the router level. This probably means that you'll have to configure your router to log into your ISP's network (so you'll have to remember whatever your ISP username/password is).
Even though this explained the same problem I was having, the resolution didn't involve "Double NAT". The solution was a simple check box on my D-Link router in the APPLICATION LEVEL GATEWAY (ALG) CONFIGURATION. See here for the solution to this problem for me.
Your link is to a post nearly as old in relative terms as this necro thread you've resurrected. Do you think these folks haven't resolved their problem by now? Do you by any chance have financial interests in that website?