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 Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
- Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
- TCL Roku TV 43S425/C107X, Ser X000001R60KV, wifi