If you want your Roku Ultra 4850 to use wifi 6, you can try configuring your 5ghz band to use "AX" protocol under the Wireless Mode settings in your router.
For testing purposes, you can try creating a new SSID (if you have a triband router) and limit it to AX-only devices on the 5ghz. (this should help confirm the Roku device can successfully connect and use wifi 6 speeds).
Just an FYI, wifi6=AX protocol.
I think the implementation of the 6ghz band capability (wifi6e/AX with 6ghz band and wifi7/be) is probably a ways off for Roku devices. With the 6ghz band, the client devices generally have to be much closer to the router (or additional AP/access points used) for a reliable connection.
With the mesh router system, you will likely notice that many 6ghz devices will routinely swap between the available bands (and generally settle on the 5ghz band for longer periods of time) as the signal decreases in an effort to remain connected to the network.