If you are unable to connect (or even find) any of the Roku devices with the Roku mobile app, verify that your mobile device (phone) is using wifi and connected to the same network as your Roku devices. (make sure you are not using your phone on the phone data, but actually connected to wifi network).
If you have verified the above, your router or network may have in place some AP isolation rules preventing the discovery of your Roku devices. (Look for something in router called AP isolation, Wireless/Wifi isolation, Network or Client isolation). Additionally, verify you are not connected to the Guest Mode of your network as isolation rules are generally enabled by default.
Feel free to post back with model number of router if still having issues.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941