If your VM hub is in modem mode (as hinted as in your opening post), then it isn’t broadcasting any WiFi at all, and everything is being done by your Nighthawk router.
VM’s customer services people might have told you that there was an issue in the area, and indeed there might have been (don’t bet your life on it though*), but even if there was, this can, and would, have absolutely no connection to what WiFi bands are being transmitted.
Assuming that the hub is, indeed in modem mode, then by necessity, your devices are ok connected via your Nighthawk router and are blissfully unaware that the VM hub even exists - the issue is with that, not the VM hub!
*I’ll take a guess here, having explained to the ‘expert’ on the phone about your WiFi issue, they had absolutely no idea what you were talking about, it not appearing on their pre-set menu and list of options, so they did the, not uncommon, response of ‘just make something up’! Gets you off the phone, they chalk up another successfully closed call, all’s well in the world! Well except for your devices, but who cares about that? Certainly not VM!
