I am connected to a wireless network managed by a large company, how do I change my NAT settings?

I am connected to a wireless network that is password protected, run by a company that supplies wireless internet to my barracks. I have my xbox 360 connected via ethernet from my pc, to my 360. I have no problem getting connection to my 360, but I am having trouble with my NAT settings. I have added ports udp88 udp 3074 and tcp 3074 to my firewall list, as recommended, and I am still encountering problems. I am wondering how i would manage my NAT settings if i cannot control my modem/router? i contacted my ISP and they tell me "we dont deal with 3rd party connections and we will not alter settings for gaming consoles. I have used this connection in the past on my old laptop, which is in a slag heap now, and it worked fine then. Is there something i have missed? I can connect to xbox live but my NAT is set to moderate. This causes complications loading multiplayer matches on every multiplayer game i have. I have tried most recommended steps, but nothing works so far. Is there something i missed?