This might help some people...
After I almost gave up... I came across my modem's NAT Configuration... The ports weren't open in my router...

So, I set it up so that Jazz 2's ports were open and forwarded to my router. Then, turned on port forwarding in my router, and then, I could host for longer than a minute again.

For anyone else this may concern...

(This is assuming that all Westell Modems are the same in interface)

1. Connect to your modem's browser interface, by default 192.168.1.254
2. Click on expert mode, then click on "yes."
3. Click on configuration, then under that, click on NAT.
4. Now, click on "Define Custom Service"
5. Choose "Port Fowarding Ranges of Ports"
6. In the Service name, put Jazz2 or something, and in the global port range, put 10052 - 10054, in the base host port, put 10052.
7. Choose TCP, under protocol.
8. Now, click on "add."
9. This time, fill all the boxes with "10052."
10. Under protocol, choose "UDP"
11. Click next, then click close.
12. Now, in the drop-down list, choose *Jazz2 (or whatever you named it.)(an * is placed beside all custom ports)
13. Now click "enable"
14. Under the host device list in the pop-up that appears, choose the MAC address of your router (normally, it's just a long string of numbers, usually the only one, it probably *won't* contain dashes) or if you're only one computer, choose your computer's name.
15. You're done! Try hosting now.

I haven't tried seeing weither you can join through this, but my guess is that it'd probably work.

EDIT: Apparently not. For some reason, you can only host through this.
EDITEDIT: Apparently so, as it will SOMETIMES work, but hosting appears to ALWAYS work.