upnp is called universal plug and play and it should automatically foward the ports to the router, i used to have the 8 port version of that router and it kinda got flakey and i had to get rid of it, have you tried playing 360 directly connected to your cable modem. see if people find you as laggy or whatnot, cause if they don't then you might have to replace your router. sometimes if they are old they just get weird and flakey, like guyzea's mom.
enabling uPnP on the old router i used to have BESFR81 made the router get upset and have some issues... replaced with DGL-4300 and havent had a problem since.
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