Either change your ‘new’ network’s SSID and password to the ‘old’ network’s, or temporarily wire a Sonos device to your new router with an Ethernet cable, and go in to the network settings of the Sonos controller, and tell your speakers the new SSID and password.

All of this is covered in the new router - installation instructions FAQ.

If you are using a new router, change the network name and password to same as the old router.

Hi

Follow instructions in the link below. Let us know if you are still having issues. 

https://support.sonos.com/en-us/article/connect-sonos-to-a-new-router-or-wi-fi-network

Edit: Looks like @Airgetlam and @GuitarSuperstar type faster than me.