You didn't mention what Roku model you have. You should always give this information when asking for help because sometimes the answers will depend on what model you have.
For the Roku app on your phone to work, both the phone and the Roku already must be connected to the same local net. If the Roku has lost its net connection, which may happen sometimes, you cannot use the app on your phone to set the Roku's net connection up again because the phone can't communicate with the Roku -- you must have an actual remote for that.
See if you can get the Roku connected again via means that don't require communication with the Roku:
- Restart you network router.
- Restart your Roku by disconnecting its power plug for a few seconds, then plugging it back in.
If it's still not connecting, re-establishing a connection will require a compatible remote.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
