Welcome and thank you for posting here in the Roku Community, @anudhs!
Thank you for posting your queries regarding the alternatives to using AirPlay. We'd be glad to be of assistance.
The Roku streaming platform currently doesn't support using websites. However, you can directly reach out to Apple's customer support and ask them why are the subtitles not supported when streaming via AirPlay.
Furthermore, you can also reach out the Flixtor's customer support and let them know you'd like their app on the Roku streaming platform.
Hope this helps!
Best regards,
Carly
Carly Y.
Roku Community Moderator