I see no way to delete a Roku device from the Roku mobile app.
The app should find any active Roku on the same network as the mobile device running the app. Be sure the "missing" device is plugged in, powered up, and connected to the net.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Do not ask me 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.". Please only do this if it ACTUALLY solves your problem.
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X | Streaming Stick 3600