@Criglet
Not a MacBook owner but seeing some instances online where the IMac firewall may be a culprit.
Try turning it Off. (System Setting/Network/Firewall/[Off])
If still having issues, try performing the test of using an ethernet connection with the firewall still set to Off and see if able to connect. (If so, then you need to start looking into both your MacBooks and see if you can spot any difference in the available settings that may be contributing).
Most complaints similar/mirror this https://discussions.apple.com/thread/255885211?sortBy=rank
Would also go into the Roku Ultra and go to Settings/System/Advanced System Settings/Control by Mobile Apps/Network Access and change to [Enabled or Permissive].
The [Permissive] setting should be a bit more conservative setting than the [Enabled] if their descriptions/behavior is accurate.
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Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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