Poor Speed & poor Reception

I purchased two AC1200 wifi extender for my 1800 SF home, abour (4) months ago.  The home has three levels and the spectrum Wifi router was not providing broadband to areas that need Wifi for Nest thermostats, tankless HW heaters, outdoor lighting, etc. The two installation locations continued to demonstrate poor coverage and often loose internet connection. Frustrated and getting angrier by the minute, I reset equipment, moved the extender closer to the source and continue to have problems. The symbal on my phone, although would take nearly a minute for my phone to switch sources when moving through the house showed a three bar signal..  so i though every this was in order.  But finally, i tested the speed using a phone App. and to my complete surprise, the speed was barely moving the needle. I moved the extender into the same room as the router and not much better.  My router speed is 399Mbps and the Extender in the same room is 3.78Mbps.  How is this possible??