Hi @Daaaaaved, welcome to the Sonos Community!

By reset, do you mean powering of your Era 300s and Sub or do you mean Factory Resetting? If the latter, I would advise against further resets as this will remove valuable diagnostics information that we can use.

Before any troubleshooting, I’d recommend making sure that you’re playing something that makes use of the surrounds. If you watch the news for example, it wont make use of the surrounds and Sub. If you use Netflix or Apple TV, then try some atmos content, otherwise an action movie on your regular channels should still give you 5.1 surround.

In the Sonos app, do the Era 300s and Sub show a red dot? What happens if you temporarily separate the Home Theatre setup and try to play directly to the Era 300s, does this allow you to play to them? While you haven’t changed anything on your Wi-Fi, there is always a chance that something has automatically changed on your network or an issue that was always present but mild has become worse.

After checking the above, I would recommend you check our support article regarding our system requirements. If you have any unsupported networking equipment that is listed on that page, then power it off and test use your Sonos system again. 

You’ll also want to make sure your system isn’t suffering from wireless interference. Move any sources of interference listed on our article further from Sonos and make sure other electronic devices are at least half a meter away from your Speakers. 

I hope this information helps!