Is it possible to configure the RE315 extender to use the powerline backhaul and behave as another node on my Deco P9 network the way the TP-Link website seems to suggest?

Yesterday, I decided to expand my home network signal by adding more nodes.

I went to the TP-Link website and learned that all TP-Link products work together.  I also learned that my Deco P9 network uses a "Powerline backhaul", while other TP-Link mesh networks use a different mesh method.  To me, this meant that adding a set of M4 nodes would probably not help much.

My house is in Florida and has block exterior construction.  Some of my devices (cameras, pool pump, sprinkler controller, garage door opener, etc.) are outside, or at least not inside the main house.  My house has previously been very hard on wifi networks and I've been happy with the Deco P9 mesh for its reliability indoors, but once I step outside, all bets are off.

A set of three M4 nodes for ~$130 looked like they wouldn't work, but I noticed that the TP-Link AC1200 WiFi extender was inexpensive and searching TP-Link AC1200 showed apparent compatibility with the powerline backhaul links.  This looked promising.

So, I ordered three RE315 extenders to put in my garage and back bedrooms.  My goal was to extend the mesh to work better in areas with low signal.

Amazon delivered same-day and I opened one of the RE315 extenders to begin working with it.  The first thing I noticed is that the instructions sheet wanted me to download a new app: "Tether" in addition to my existing app "Deco".  This was somewhat annoying.  Why couldn't TP-Link have one app for both devices? 

The next thing I noticed is that the documentation talked about the extender using <network_name>_EXT as its network name.  I was very unhappy to see this.  I previously dealth with _EXT extenders and had no end if difficulty with the effort of merging both networks.  I don't want to deal with that again.  However, I saw that in the Tether app, there is a "Mesh" menu option and my hopes rose that I could bring them into the main network using this feature.  Maybe this is the way to get them to use the powerline backhaul.

Nope.

Or at least, not so far...

The RE315 extender doesn't show up in my Deco app as another node.  It shows up as a device on the network.

Is it possible to configure the RE315 extender to use the powerline backhaul and behave as another node on my Deco P9 network the way the TP-Link website seems to suggest?