Extender reconnects to original SSID in network list

When I set up the extender it created a network name with an _EXT extension of my router ssid (ie Mustang1 became Mustang1_EXT) on the extender.  My devices show all networks available as usual on any laptop.  I have a laptop and firestick and set both up to the _EXT network and connected to it.  My devices are defaulting back and reconnecting to the original Mustang1 network on the list instead of maintaining connection to the _EXT network after a period of inactivity.