Since you mentioned that your router is restricting IP discovery, try looking for a setting in your router (or mobile app for your router if applicable) for a setting called "AP isolation" and disable it. Normally located under Advanced or Professional (Asus) settings.
The above setting, when enabled, will prevent network connected devices from discovering and communicating with each other. (ie. mobile apps and Roku).
In addition, check your Roku settings and verify that your Roku device allows mobile app access.
Settings/System/Advanced System Settings/Control by mobile apps/Network Access/[set to Default or Permissive].
Feel free to post back with router model number if still having issues.
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