I've had the same problem. Sometimes my RokuTV shows up on the app, sometimes not. My other Roku device shows up every time.
All my devices are on the same subnet, network, router, etc. Actually the Roku device that does work is older on the 2.4 Ghz and I see that all the time. My other devices on the 5G....
Oh, as I wrote this, I think I found the problem. The RokuTV doesn't like the 5G. When I put the RokuTV on the 2.4Ghz it was almost instantly found on the Roku App.
I had to separate the SSIDs on my router so one was for 2.4Ghz, and another was for 5Ghz. With the RokuTV on the 2.4Ghz, it seems to work fine now. My other devices can be on either 2.4 or 5Ghz.
Might suck as this is a 4k TV and doubt it can get 4k over the 2.4Ghz.
One more thing that I think helped was to disable ASUS "airtime fairness". It seems to disconnect lower usage devices randomly.