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Please note that the Roku mobile app will work only when the Roku TV and the phone are connected to the same WiFi network.
Here's a link to our support page to find out more about the feature: How to connect the Roku® mobile app to your Roku device over Wi-Fi® | Official Roku Support
However, if there are any changes on the network (SSID and password), you will need a physical remote in order to input your new password and connect to a new wireless network. For US customers, you can check out replacement remotes in our accessories store here
Hope this helps! Let us know if you have any other concerns.