I had this working fine before my new modem showed up today. I was able to setup the chromecast ultra easily. Trying to setup the audio, I can find it, I can play the test audio but when I try to connect to the network I get the message "cannot communicate with chromecast audio". Over and over. I have tried to manually enter the password and let it do so automatically, with the same result.
Hey there Kayakidiot,
I'm sorry to hear that your Chromecast Audio is having some issues with your new modem. Networking issues can be pretty tricky to troubleshoot, so I'd be happy to help out. First, what is the status of the Chromecast Audio's LED light? This could point to an error that the device is having. If everything looks fine there, I would check out this article on Wi-Fi connectivity issues.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!