Note that 5G (mobile network) is a different thing that 5Ghz channels of 802.11b/g/n. Sonos does say in their Sonos Network Requirements FAQ that they don’t support mobile networks, so you may be challenged when calling in. 

Where I’d be looking is to see if this 5G router has a maximum number of connections….each Sonos speaker will require one, and it’s possible that the router is maxed out. 

One way you may be able to get around such a restriction is to set up your own “normal” router connected to the 5G router. The 5G router will supply the internet signal to the normal router, which then  sends the signal on to Sonos. Since Sonos can’t “see” the 5G router, it has no idea where the signal is originating from. You’d want to set up the 5G router in “bridge” mode, though, having two devices with DHCP (network assignments for devices) could be an issue. 

@Rich1012,

It looks to me like the routers WiFi ‘backward compatibility’ and/or ‘security mode’ setup on the 2.4Ghz band is not compatible with Sonos with its current settings - log onto the router and if necessary set that band to support 802.11b/g/n and use ‘WPA2 personal’ security mode. See if that resolves your issue.

I would also remove the underscores from the SSID name too, in case that’s causing an issue.

I suspect the 2.4Ghz/5Ghz bands are combined in this instance, but  that (hopefully) should not matter, but the WiFi backward compatibility and/or security mode may just (possibly) be the problem here.