Without knowledge of which new modem/router it is, it’s hard to be certain, but I’d try a simple network refresh, by unplugging all Sonos devices from power, then rebooting this new router/modem. Give it two minutes to reload, then plug back in the Sonos devices, and give them two minutes to boot up ands load, then check for any updates, and apply them as needed, and finally test your music streams. 

For more direct assistance, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, 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.