Just today I tried to use the iPhone Roku remote app and it can’t find my Roku Express (3900 series) . It was working fine up to a day or so ago. It also no longer shows the Express as a device I can cast to from the YouTube app. I’ve not done anything to my internet router. They both have the same first three sections of their IP address. Even when I try entering the IP address manually reading it off the settings screen of my Roku express it still says it can’t find it in my network. It is connected fine as we can watch things on the TV. Just it refuses to show itself to the phone or vice versa
Try restarting all three things involved in the connection. (Your phone, the Roku device, and your router).
And of course, make sure your phone is connected to your wireless network and not inadvertently switched to your data connection or another wireless network.