I am having exact problem mirroring/casting from Galaxy S20, but to a 2018 SAMSUNG smart TV. So, it's obviously not a brand specific issue.
The only fix I have had luck with 2 out of 3 time is connect to Roku via smart view on phone. Once it connects and u only have sound (no video) turn wifi on you phone off which will disconnect you from TV via SmartView. Now go back to smart view and try to reconnect with wifi still off. If this doesn't work and usually it doesn't. I turn wifi back on and try connect thru smart view to TV and usually the video works. Consequently both me and my wife have this problem on Seperate phones and once one phone connects the other usually will no problem.
Thanks for letting us.
When you want to display content from your phone or tablet on a nearby TV, you may be able to use screen mirroring. To use screen mirroring, you must first set up and enable the feature on your Android or Windows device and then request a connection to your Roku device.
Did you reach the process where your Roku device asks you to accept a request for a screen mirroring connection to be established? Could you walk us through the steps to reproduce the issue you are seeing? What troubleshooting steps have you already taken to try to resolve the issue?
For full information on this, you can take a look at this link: How to screen mirror your Android or Windows phone to your Roku streaming device
With more details, we will be able to assist you further.
Regards,
Nimfa