Reposting this question as the answer involved setting chromecast audio with an old device. This is not an accepatble answer for me as I do not have an old device and this is not working with my Pixel 3a.
Is it possible to use an old Chromecast Audio with Google Home currently?
Device Utility finds it but then says it can not connect to it. Home finds it and says it needs to activated first.
I can connect to it's WiFi. Is there a way to set it though the Chrome browser? What is the ip that needs to be opened to access settings?
The best thing to do here is follow these steps:
Setup can only be done using a phone or tablet, and can't be done using a computer.