First off, I wouldn’t ’factory reset’ the devices without Sonos telling you to do so.

I’d be tempted to temporarily wire the Arc Ultra to the router with an Ethernet cable to see if it connects to the controller at that point. If it does, I’d make sure the WiFi/radio is turned on, and likely flip that ‘switch’ in the controller a couple of times, just to be sure it is on. The Arc connects to the Sub via a ‘hidden’ network connection, so that WiFi needs to be ‘on’ on both of them. 

I’d also be tempted to power cycle both the Arc and the Sub. Shoot, I’d also be tempted to reboot that router. It’s certainly possible, with any electronic device, that they were impacted by a power change (commonly called a surge). Different devices can certainly have different reactions to those kind of events. 

But, if none of that works, I would recommend that you submit a system diagnostic, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for the info, I'll be giving it a try tomorrow and see how it goes.

I’d wait a minute before taking any action.

I’d think ‘tomorrow’ qualifies as ‘a minute’…

Thanks for your advice - I haven't done anything as it seems to have "righted" itself overnight!