While we don't offer WiFi extenders ourselves, we have tested the TP-Link WiFi extenders and found them to be very effective at improving the speed and signal of the networks in larger residences/buildings. These can be purchased relatively inexpensively from retailers and are easy to set up with the routers we provide.
TP-Link WiFi Range Extender Line-up (Japanese):
https://www.tp-link.com/jp/home-networking/range-extender/
Specific Model Tested: RE700X - AX3000 Mesh WiFi 6 Extender
https://www.tp-link.com/jp/home-networking/range-extender/re700x/