I have just purchased (Sept 2020) a Deco Mesh Router System via Amazon (US).  Main Router Model is Deco M4R and Two (2) Plug In Extenders Model Deco M3W.  They  connect (via Ethernet) to my newly acquired Fios Router https://www.verizon.com/support/residential/internet/equipment/routers/fios-router with a new plan of 400/400 Mbps Up Down.

1) I live in a split level house approximately 1500 sqaure feet.  First floor is a basement floor with a lot of brickwork around the foundation-perimeter and a lot of forced air/heat ductwork in the ceilings.  Second floor is a half-level up and the second floor is "separated" or divided from the lower first floor by the first floor's brick foundation-perimeter.  The third floor is another half level up from the second floor.  The third floor sits "atop" the first floor footprint and thus is separated from the first floor by the aforementioned forced air-heat ductwork.

Graphically it kind of looks like this

                                |                                               |

                                |                                               |                                                             |

                                |___________Third Floor_____ |                                                             |

                                |                                              |____________Second Floor__________|   

------- Ground ------|                                              |--------------------------------------------------- Ground ------

                                |__________First Floor_______|

My FIOS router is wedged into a small space/closet shelf right about where the first, second and third floor meet in the graphic above.  BUT, the FIOS router is on the first floor side of the brick foundation wall between the first and the second floor.

I was planning on hardwiring (Ethernet Cat50 the Deco M4R to the FIOS router and placing it in the the First Floor.  Then having one Extender Model Deco M3W in the far end of the Second Floor and the second Extender Model Deco M3W in the far end of the first floor (in a home office right next to the office computer using it the most which needs the "best" signal and speed.

Question:  Is this a good setup or should I readjust things?

2) Should the Main Router Model is Deco M4R be in "Router (Default)" mode or "Access Point" mode.  The Deco App seems to describe my setup such that I should be in "Access Point" mode.

3) I've seen indications in other threads that I should shut the FIOS's WIFI signal off.  Please confirm?

4) I've also seen indications in other threads that I can use an app (the Deco app?) to "measure" the speed and reliability of the connection between my Main Router Deco M4R and the two Plug In Extender Deco M3Ws.  Please explain how to do that?

Thanks.

JSJS2