OgRoar
What are the model numbers of the non-working Roku devices and what is the model number of the router?
If this router is capable of wifi6, try configuring one of the 5ghz bands to use only n/AC under the 'wifi mode' or 'wifi protocol'. Then configure the 'security' (wifi password) to only use WPA2-Personal. (do not use/include WPA3 or any 'compatibility mode' options). Additionally, try only using numbers and letters for the wifi/ssid password. (avoid using special characters).
Also, if you are using any router/ISP apps to control your network, make sure you didn't accidently block/pause any connection of the Roku to access the network or internet.
See if making the above changes helps with connecting. With the router model number, may be able to see if any additional options are available or may be contributing. Also mention if you have any band steering or mesh enabled. (a few routers will still allow you to name the SSIDs yet still manage the connections so not sure if a mesh setting is contributing).
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Just to clarify, you have 5 Roku devices. It was mentioned that 'all 4' connect to 2.4ghz fine. If you can provide a list of all your Roku device model numbers (can get them from Settings/System/About), and specify what bands they are connecting to (or are successfully connecting to) this may help narrow down issue. (network capabilities of these Roku devices can vary among these devices, with band capability/determination only one piece of it).
