I've still been having this exact same issue. My device and tablet are confirmed to be on the same wi-fi network and same frequency, but it's like the app can't seem to communicate. I also don't have any blocks on my phone or tablet that world prevents communication. Restarting my device does not make any difference. The mobile app used to work just fine.
What's worse is that my actual Roku remote went missing, and just had a brief power outage. The roku lost power, and when rebooting, it did not automatically reconnect to the wifi network. WITHOUT the roku remote, I cannot connect to use the Roku at all.