Best answer by Airgetlam
Many of Sonos' devices were designed when 5Ghz wasn't a "thing", so some of them don't have the ability to use 5Ghz at all. Also, 5Ghz doesn't have the same range as 2.4Ghz, so Sonos (I suspect) has stuck with 2.4Ghz. But the point at which they made those decisions and now are certainly different, but they've tried to maintain as much comparability as they can.
There's 2 possibilities if the PLAYBAR does show up when hardwired to your router (and there's no reason that you can't move it temporarily for this process). One is that your network is only 5GHz, and two (much less likely IMHO) that there's a bad wifi card on your PLAYBAR. Not completely impossible, just extremely unlikely. I'd guess 99% chance of the first, not the second.
If it does show up when connected, I'd submit a system diagnostic and contact Sonos to discuss it. They'd be able to read in that diagnostic if in fact there is a failure, and probably be able to help you with the wifi as well.