@Bami 

Reference the LED lights on the extender to verify if it is actually connected. If your Tether app does not see the extender, then it may not be connected to the router. I would recommend factory resetting the extender (hold the reset button while plugged into the outlet for 15 seconds), then using the Tether app to set up the device. That way you can name the extender's extended network to be what you would prefer it to be: https://www.tp-link.com/us/support/faq/1373/

@Tony  Thanks Tony.  Actually, it just took about a day and the extender became visible on the Tether app and is working well.  One question though - when looking for the router or extender on any device I only see one name - the name of the network.  I don't see the extender separately and I don't see 2.5 and 5 g separately on the router or the extender.  This is actually more convenient and seems to be more a real mesh network this way, but I just wanted to be sure that this is normal and doesn't indicate that I've set it up wrong.  Thanks again.