TP-LINK RE210 and TP-LINK RE300 Will Not Connect to Xfinity TG02DCW3482P2CT

I have both a TP-LINK RE210 and TP-LINK RE300 and despite repeated attempts to connect them to the Xfinity TG02DCW3482P2CT - they will not connect. I've tried everything, from resetting both TP-LINK devices to factory defaults and re-setting them up - nothing.

I tried the TP-LINK devices connecting to a NetGear Orbi, and had no issues whatsoever, it worked just fine.

Is there some sort of a setting on the Xfinity device which is preventing the TP-LINK devices from attaching?

Also - FWIW, I cannot get my ecobee thermostat to connect to the Xfinity Wi-Fi either. 

There is something wrong with the Xfinity device - what settings would not allow standard extenders to connect?