I have three Chromecasts set up to my router. The two regular chromecasts connect to the wifi fine. However, I have found that my Chromecast Audio will not connect to the wifi anymoreI have tried setting up again but it suggests I disable Access Points on the router.
I have searched the Google forums and it seems there was alot of problems with Audio Chromecasts not connecting to Plusnet and BT Hubs some time ago. This problem only seems to have arisen when I had to reset all my chromecasts to my Google Home app.
The router is running firmware 184.108.40.206.220.127.116.11.2 (which was last updated on 11/08/17), but the chromecast was working up until December 2018.
After 3 hours of searching I found a solution (not ideal but it works).
1. Enter the router settings and go to advance settings > wireless > 5ghz wireless
2. Sync with 2.4 GHz set to No
3. Change the Wireless SSID in the box below. (I just added a number to the end of the SSID set by Plusnet.
The Chromecast now works the only downside is I now have two separate wifi networks one for 2.4ghz (orginal plusnet set wifi network) and aone for 5ghz (the plusnet name with the new digit at the end).