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What error message are you receiving when you use your chromecast? Have you also tried to reset everything and re-setup it?
Are you able to provide more information for us to analyze what is happening with your chromecast connection?
Looking at my desktop with Windows 10 can’t seem to see the Chromecast or the Google Home Mini showing anywhere although not sure I’m sufficiently skilled to do so. The home mini took to the NBN and new modem router seamlessly and is functioning ok. iPhones and IPads also connect to WiFi ok.