"There is more to the problem. Samsung has deliberately disabled landscape mode in their camera app on Note 9 and 10 and I believe all S series from S8 on when used with screen mirror or HDMI connections. This seems to have occurred coincident with the introduction of DEX. I use an external HDMI monitor when making videos ALWAYS in landscape mode just like the pros do. This allows me to put the phone on a selfie stick and film over the heads of a crowd of people. Nothing I have tried including attempting to get help from Samsung has resulted in a solution other than using third party camera app which seems to work albeit without the features of the Samsung app. As a retired electronics engineer I can not think of a technical reason for this behavior. As a long time Samsung user I will be looking seriously at the competition when considering my next upgrade even though it means giving up the S-pen."
"Is this going to get resolved? For those that continue to post comments "Did you turn landscaping on? <<insert hillbilly voice>>. Please read comments first. The problem is when you have smartview on and turn the (Samsungs built-in) camera app, you will get a message at the bottom that says "Can't switch to landscape view on displays connected through Smart View or HDMI". Since there is a designated error for when this occurs, that means it is an expected result, meaning, it is intentionally not working or unable to work. Only way to bypass this message and get working results, you download a 3rd party and boom you get working results. Only problem is you lose a lot of functional features from the built-in phone app. Now, we(the community) have to ask the when or is it going to get fixed?"
Nothing to do with Roku, you'll want to address this to Samsung.