Adding TP-Link RE315 extender - problems

Hello, I read and tried many of the suggestions in other posts, but some things aren't proceeding the same as my situation.  I'm trying to link up a TP-Link RE315 extender with my TMobile router, KVD21.  On the TP-Link device, I connected wirelessly thru my laptop and I went in as admin and added the SSID, pw for the TMobile modem/router and have the 2g and 5g buttons lit.  Somehow, yesterday my phone picked up the up the TP-Link wireless network, but no internet connectivity, but today it's not even showing as an available network. I've tried ethernet connecting and the Tethering app, but, though it sees the RE315, it doesn't pick up the TMobile KVD21 router as a device, even though there's an ethernet cable from between the 2 devices. 

I'm usually not inept at this stuff, but today?  I cry uncle!!  Has anyone has these issues, who can help?  ¡Gracias!