Before I bought a Deco E4, I owned a TP link Archer C9 router.

This router was connected to another Archer C9 which was in my parent's house which is next doors. This was we were on the same LAN and sharing internet as well. 

I am unable to connect my Archer C9 as a slave to the Deco E4. Most of the ethernet backhaul literature talks about connecting other routers in the Deco line-up. Where I live, they only sell 2 and 3 packs so I can't buy just one E4 and add it to my parents' smaller home!

Does anyone have any idea on how I can configure this: Internet modem connected to Deco E4R master, which then creates a mesh with other Deco routers wirelessly. Archer C9 is connected by ethernet to the Deco E4R Master and is part of the same LAN and shares the same internet.