I was having this same problem just tonight only if I was on wireless internet on Roku TV my android app couldn't see that it was both on and connected nor could it wake it up. However if I did a wired internet connection on the TV the Roku app had no problem connecting to it which was frustrating to say the least! Ultimately by reading several postings both here in the Roku community as well as several other places online and piecing together things they said and tried I got the problem solved easily which I will post here for future easy reference.
Make sure TV is connected to your wireless network. Newer Android phones and Roku TVs do not differentiate between 2.4, 5g or wired in order for for your android app and tv to work together only that all are on the same network but older model android phones, older android software versions or older roku tvs may so if you have newer models of each make sure your software and firmwares are up to date before doing this.
I have a TCL Roku TV model 49S403 with up to date firmware as of August 17, 2020 and at this time have the Samsung Galaxy S10+ with Android 10 up to date as of August 17, 2020
Go to the following: Settingings-System-Power-Auto power settings and the selected/turned on both -reduce power after 15 minutes as well as turn off after 4 hours. Then initiated a system restart located in the same power settings.
When everything including network connection (your current local time pops up) go back to: Settings-System-Power-Auto power settings and de-select/turn off both -reduce power after 15 minutes and turn off after 4 hours at which point my android Roku app instantly found my device.
At this point check that the app does indeed control TV including private listening. After confirming this completely close out the Roku app on your android (go to recent apps select close all) and initiate a system restart once more on your Roku TV and you should have no more issues!