Connect tp-Link extender to Gateway 5G

Looking at the spec sheet for that model, the ethernet port is not for connecting to your gateway. It is only used as a bridge for device(s) that don't have wifi. Connecting the extender ethernet to the Tmobile gateway will do nothing.

You must connect the the TP Link extender to your Tmobile gateway via 2.4 Ghz wireless wifi as it does not support 5 Ghz. The setup guide says to use the TP Link app from your mobile device.

Considering all of this, the question is why do you need this extender? 2.4 Ghz from your Tmobile gateway should easily cover a 3000 sq ft house at the same if not better data rate than the extender. The 300 mbps maximum spec of your extender means you will average closer to 150 mbps.

What are you trying to solve with this extender?

Run an ethernet cable from the gateway to the extender.  

Have tried that.  T-Mobile tech said to leave connected from 24 hrs. and it would be opperative.  Has been 36 and the connectivity has failed.  Any other thoughts?   It is a T-Link model TL-WA860RE.