The technique I often employ on Avalanche 5.X is to Ping the device(s) before trying to Remote Control them. If that doesn't work you may need to reinstall the Remote Control package on the devices. To trigger a reinstall of the package, edit the Client Configuration of Remote Control and change the max log size from 100 to something insignificantly different like 101. The devices will then recognize that they need to download the updated version of the package and will reinstall the Remote Control utility and relaunch the executable on the device in the process.
That appears to have done it. Thank you for your help!