There is a 186 page user manual for the Westell 7500 that I am not about to read in detail. I had one of these modems many years ago, and, as I recall, it had a very complex advanced menu system, with sections on its firewall and other security settings.
Configuring OBi devices with their WiFi dongles can be problematic. I'd suggest first trying to instead use a hardwired Ethernet cable connection between the OBi and the router, and see if that works. If so, you're wasting your time on troubleshooting security. It could just be a WiFi problem.
http://onlinehelp.verizon.net/consumer/bin/pdf/VersaLink7500UserGuide.pdf