I can assure you it's not. The way he's just set up the Tenda, it is litteraly just passing through traffic, it's not manipulating it.
not having set up a network myself with a second router in it i can only assume it's a conflict between the belkin and tenda firewall rules.
This is actually a good idea. Though my guts say that it won't matter.
so this is what i'd do, first i'd test by just connecting the switch to the belkin only (disconnect the tenda) and seeing if it then works, this will confirm the problem is between the belkin and tenda somehow.
Please please PLEASE don't turn UPnP on. There is no good reason to use UPnP, and it's super easy to just setup your ports manually. UPnP opens up a whole new can of worms.
set the belkin settings to default and make sure upnp is on then test to see if it works from there.
Doesn't matter as the tenda isn't handling the traffic.
if it does now work i'd go into the tenda settings and make sure everything that i could turn off was turned off, upnp, dhcp, etc. etc. so it's only the belkin and it's settings that control what data and ports are sent where then connect the tenda back to the belkin and switch to the tenda and test again.
DNS has nothing to do with NAT issues.
if it still didn't work just connected to the belkin then i'd guess the issue is elsewhere with that game and i'd try maybe using a different dns server in the router to see if that solved the problem.
What might also be an issue (but I have not researched this) is the service provider blocking certain ports to protect their network. I have seen this in the past.
Try connecting directly to the Belkin first, if that doesn't work, contact your provider to see if they have any closed ports.