Of course I'd test by putting a Linksys router between my system and the Westell and see if my computers can communicate, but then I've got those sitting on the shelf here.

If it is the Westell, and I've had provider supplied routers go insane on me, the problem is convincing the provider, and that's not easy. Sometimes a factory reset will fix the problem, but you have to know the things default IP address and exactly how it was set up (and how to do the factory reset).

Another thing I'd do immediately is set my computers to static IP, restart them and see if they can communicate. I never use dhcp on small networks except for notebooks that are carried around because static makes testing much easier.