Hey @Kaf, that's not good to hear. I have managed to connect my own Chromecast at home on the 5ghz channel. Were your other devices able to connect to the 5gHZ Wi-Fi?
If it was isolated to your Chromecast, I would recommend having a looking into the manual and settings to make sure everything was in spec.
Should it be impacting all other devices, I would recommend Troubleshooting your 5G connection, doing so will just get things in order and help better the connectivity from your premise
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While I don't use a Chromecast device, I had to fiddle around with 5GHz settings before it worked with some devices.
Got it working 80MHz and Channel 36
Normally I work with Ruckus or Aruba for WiFi, so it was rather strange.