i do this all the time. you have to find the setting in the phone menu to turn on miracast (might just cal it screen mirroring. depends on device)
you need to have the roku and phone on the same wifi network for the phone to find the roku. if it does, it'll pop up the roku stick and you simply click on it. it will connect and show your phone screen on the tv. then just play whatever you want using whatever player you wish and it will show to the tv. it simply shows what is on the phone so anything you do is mirrored. you can play games and such on the tv this was though it is terribly laggy. as noted above, this is not related to the roku app. it's in the phone settings somewhere.
you will get some audio lag so you want a player that lets you adjust that. my tv is -250 ms off. but yours will likely be different. i use VLC player as it plays all the files and let's me adjust the audio/subtitle timing if needed.
the roku app actually has a player built in that casts easily but it does not play very many file types. can give it a try but unlikely it will play all the video files you may have.