More than a year later and the upper 5 GHz bands are still broken/not working on Chromecast Ultra (fw version 1.56.281627). Earliest reports seem to be from more than seven years ago!?!
Crazy that this is still an issue, as using a 2.4 GHz connection gives a warning during setup that 4K content might be limited with it, and is usually more congested. Most non tech enthusiast have no idea how to limit their 5 GHz WiFi to the lower bands. Just about every WiFi accesspoint/router does automatic band selection, so a Chromecast will loose the WiFi connection if the accesspoint switches to a high 5 GHz band after a reboot.
Thanks for reaching out and for providing detailed information on your issue and the steps that you've done — these are of great help!
Could you help us with a few things? What is the make and model of the router that you are using? Also, what channel are you using for your Chromecast Ultra?
Looking forward to your response.
Sure, and thanks for taking the time.
The Chromecast Ultra is connected to an Ubiquiti UniFi AP AC Lite, running firmware 220.127.116.11.
The 5 GHz radio is currently locked to channel 48 and the Chromecast is connected. It cannot connect if the radio is set to any higher band channel (149, 153, 157, 161 or 165). Other devices (Android, iOS, TV, laptops) are able to connect on a high band setting.