I have a "Main" Deco M4R connected (via Ethernet) to Verizon FIOS. 

Wired connection to the FIOS router routinely gives me 400-500 Mbps speeds.

Deco M4R never gets near that speed via Deco M4R's wifi connection on any device (whether Windows, Apple Mac or iOS).  Standing next to the Deco M4R, I sometimes get mid-300 Mbps.  Stepping about 20 feet away with an unobstructed view of the Deco M4R, I drop down into the low 100s or much worse.  If I go one floor up from Deco M4R (about 15 feet distance, only a wood framed ceiling/floor (no conduit or ductwork). I drop down below 100s consistently.

Plug In Extenders (two) are Deco M3Ws.  All white indicator lights across showing on each.  Barely get 50Mbps when standing right next this two units.

Any way to fix this?

Worse, if I leave a room with an extender and move away from the extender into a room with another extender (or the base), I lose my connection and I have to disconnect and reconnect to the network.  Thoughts?

PS

This link

https://www.tp-link.com/us/support/faq/1446/

isn't helpful.

In particular.  On my app, this isn't right.  The "Main Deco" displays some of the information contained in the second screenshot.  But, the "Living Room" (one of my Deco M3Ws is actually named Living Room) just says "Deco Health Check.  Deco is working well."  It doesn't display the speed, or signal source strength.