What browser are you using? HT20/40 should not be grayed out..and should be selectable...
Web Browser Configurations
What browser are you using?
Try Opera or FF? If IE 8, 9, 10 or 11, set compatibility mode and test again.
Disable any security browser Add-ons like No Script and Ad-Block or configure them to allow All Pages when connected to the router.
Clear all browser caches.
Try turning off these features in Chrome:
Top right corner, little bars for options > Settings > Settings (on left) > Show advanced settings.
Uncheck these:
Use a web service to help resolve navigation errors
Use a prediction service to help complete searches and URLs typed in the address bar
Predict network actions to improve page load performance
Enable phishing and malware protection
What FW version is currently loaded on the router?
What is the distance between the router and XB1?
Please test the XB1 connected to the 5Ghz radio, single mode N, WPA2/AES only.
The SB 6120 is a good modem. I have this same model mode and works well.
One thing, I recommend that you have your ISP check the cabling going to the ISP modem, check signal levels going to the ISP modem. Check for t.v. line splitters and remove them as they can introduce noise on the line and lower the signal going to the ISP modem. Connecting to the ISP modem could result in a false positive as the signal to the modem could be just enough to that point then adding on a router, could see problems. The router operation is dependent upon getting good data flow from the ISP modem and the modem is dependent upon getting good signal from the ISP Service.