@Hiraeth 

We haven't had any similar feedback before.  Have you tired connecting directly to your modem to see if you can replicate the issue?  Also can you do the following:

go to the router's interface click on the Advanced> Network> Internet> Advanced> Use the following DNS addresses. 

Then type in Primary:  8.8.8.8 Secondary: 8.8.4.4.  Click save and reboot the router then test again and see if the downloads work. 

@Carl 

Okay, so I did as you suggested, and still no Windows Update, which also means no malware definitions from Bitdefender as well - don't like that at all.  While I was connected direct from the PC's network adapter to the cable modem, I tried connecting to Windows Update and while I could pull the site up, when I searched for updates, the search eventually timed out reporting back an error code that I couldn't trace down in a brief online search.  This leads me to believe it's a MAC address problem.  Changing the DNS server addresses didn't affect anything, so I've changed them back to what they were originally presumably assigned during the DHCP process.  My next stop is the helpdesk @ my ISP, I suppose.  I can't be the only customer who's attached a wi-fi router to one of their cable modems.

@Hiraeth 

Let us know how it turns out.  I have seen in the past where traffic routing was to blame.  A hope in the route is blocked or flagged causing specific traffic to break.  That is why we suggested using Google's Free DNS servers, that usually fixes the issue and they are usually more reliable than the ones supplied by the ISP.