Your router may be using DFS channels (52-140). Roku cannot see or use these DFS Channels. As such, Roku can use ch. 36-48 (low band) and 149-161 (high band). It is also possible that your Bandwidth/Channel Width may be set too high causing the disruption/disconnection.
From within the 5ghz Wireless Settings of the router, (normally under Advanced for most routers), see if you have an option to enable/disable DFS channels within Auto Channel Selection, or the like. You will want to select "Disable". Alternatively, if no option is present, you may want to try manually assigning a wireless channel rather than using "Auto". (Try ch 149 initially).
Additionally, you can try lowering your Bandwidth/Channel Width to 40mhz on the 5ghz band. 80mhz and higher may cause disconnections depending on network setup.
When you get disconnected, you can try navigating in Roku to Settings..Network..Setup Connection..Wireless and selecting "Scan Again to See All Connections" and press a few times to see if that refreshes the SSID Network Names (your 5ghz connection name).
Feel free to post back with an update or if you need more help.
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Re: 5ghz appearing, connecting, disconnecting and never returned
Along with your input from @AvsGunnar, your router may be set to automatically select the channel it will use. If yours is set that way, and if it can use the DFS channels that Rokus cannot use, that could explain the Roku's ability to see your network coming and going depending on which channel it has currently selected.
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