Cable one Sonos device only to your main router and wait a couple of minutes for all products to Appear in the Sonos App.

You mentioned you have reset the App, but hopefully the devices will all appear, but try rebooting any that don’t and if necessary reset your App again and connect back to the ‘existing system’, but definitely do not factory reset any actual Sonos device.

When you have all devices showing in your App goto “Settings/System/Network/Manage Networks” and delete your old WiFi credentials and then select “Update Networks”.

Follow the onscreen instructions to Add your new WiFi credentials (SSID/Password) to all your Sonos products. When completed, you can then uncable the wired speaker from your router and all devices should then run on your new routers WiFi - note it may take 5 minutes, or so, for your devices to settle.

Thanks for your reply. Attaching the device via a cable is required? My router is in a different part of the house and it’s a pain in the backside to unhook the playbase (it’s not small) to get it close enough to attach it via a lead. 
 

Seems very off that the app would grey out the systems page so you can’t get in and change settings

It might just be the case that there is a Sonos update and/or a mismatch between the Sonos App and the firmware version on the speaker, but you need to get your device back onto the LAN/WAN first to update it and alter its WiFi settings.

Do you not have any ‘other’ Sonos devices to temporarily cable to the router, or an access point (with ethernet port) you can temporarily move near to the PlayBase?

Some users occasionally choose to change their new router settings so that they are the same SSID/Password as their old router and then do the updates/changes mentioned, but that’s upto you. 

I was trying to keep things fairly straightforward in my earlier replies as usually it’s often easy to cable one device to the LAN.