Anyone have the sequence for Roku Uktra?
I did find this elsewhere and just confirmed it with my Roku Ultra:
Home 3x (just to make sure you're safely planted at the "Home" option on the home screen)
Up 1x (Cursor is now on "Settings")
OK 1x (You're now in the settings menu...you could also press right 1x, same effect)
(This is the change needed for Ultra. Other Roku would be down 3x. Cursor now on "Display type")
OK 1x (or right 1x, now in the Display type menu)
Down 1x (cursor now on 720p. This assumes your Roku reverted to "Auto detect." If it isn't on Auto detect, pressing down once will put you on a different resolution option)
OK 1xWait about 5 seconds for it to check this setting.
OK 1x (Accepting option "OK, go to 720p")
Then it will make the switch, and you should see the "Is the screen displaying correctly?" prompt after a few seconds. If not, the default option is "No, I will choose a different setting," which you can click OK to accept, or it will default to that within 10 seconds. Click up 1x then OK to accept the setting, though this is probably a bad choice if you can't see the "Is the screen displaying correctly?" prompt.
It's really annoying Roku hasn't created an official way to reset the resolution, seeing as auto-detect clearly has issues. Perhaps you can hold a certain key for 20 seconds and it will try different resolutions until you confirm one is working, I don't know.
For me the problem occurred because I have a 4k TV connected to a receiver that doesn't support 4k, so auto-detect will sometimes set it to 4k.