Hello.
I just downloaded Roku's iOS app into an old iPhone 6+ (iOS v12.4.6). I got it connected to a 28" TCL HDTV (model 28S30S; Roku TV: 8101X; software v9.2.0 b4502-48) easily. However, I am unable to type in English texts in the HDTV's YouTube app's search with iPhone's onscreen keyboard input. I had to to enter texts by navigating each character one by one like a RC which is too slow. It works fine from HDTV's Roku's home screen's and Vimeo app's search screens. Why not in YouTube app?
Also, I cannot use iPhone's Chinese keyboard to enter Chinese characters like with Apple TV 4K and its Remote iOS app. Does Roku not support Chinese characters inputs? 
Thank you for reading and hopefully answering. 
Ant @ http://aqfl.net, http://antfarm.home.dhs.org, & http://antfarm.ma.cx.