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I'm assuming by standalone broadband box, you mean a cable modem rather than Set Top Box. I also assume your router is cable compatible i.e. does not include any inbuilt ADSL modem.
Start by establishing a wired connection between your modem router and PC. Later you can set up the wireless.
Please do a reset to factory settings on the router to wipe any settings you may have inadvertently tweaked.
Procedure for connection is:
1) Power everything (modem, router, computer) off.
2) Connect ethernet between modem and router (WAN) connect ethernet between router (LAN) and PC. (no other USB or ethernet lead to be connected to router)
3) Power modem and let settle, power router let settle, power PC.
In theory that should be it. If not we need to trouble shoot.
A) Can you browse from your PC to the router's admin page. If not you need to check the cable between the router and PC, and the PC's network settings.
B) Are the modem lights lit. If not try a different ethernet lead between the modem and router. If that doesn't solve it then, since your PC can get a direct network connection, it must be an error in the router configuration.