You didnt mention your Roku TV brand/model.
Need a lot more information, especially about the network configuration (wired or wireless, home or school/hotel, etc).
Your Roku TV and your phone/tablet must be connected to the same network, and without any AP/Client Isolation active (so that devices can see/connect to each other).
Make sure both your Roku TV and phone are actively connected to the network - if this is a school/hotel network, it likely has AP/Client isolation enabled so your devices wont be able to see each other.
