I need some help with a wifi configuration please, as the 3rd floor of our house is struggling.

The router is on the ground floor. I have 2 x RE650 range extenders. I brought them both down to the ground floor and paired them to the router using WPS. I then relocated one to the 1st floor and the other to the 2nd floor, but I get many periods where there is no connection at all.

I do not know Hardware Version or Firmware Version; Tether doesn't show that information. The box from the extenders shows AC2600 Dual Band 1733 Mbps 5GHz and 800Mbps 2.4GHz

I do not have 5GHz setup at all on the entenders, as I was under the impression that 2.4GHz worked better indoors...between floors etc. 

So have I done things correctly with both extenders paired to the main router on the ground floor, or should the extender on the 2nd floor be paired with the extender on the 1st floor?

Should 5GHz be enabled on the extenders, rather than disabled?

Help...struggling to get the ideal setup for seamless wifi in this 3 level rental house

cheers