The phone/tablet running the Roku app must have its wifi turned on and be connected to the same home network as the Roku TV.
Some network routers may have a setting that prevents different devices from "seeing" each other, especially if on a "guest" network setting.
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