Step one: Optimize what you've got

• Get routers/single wifi up and out in the open. Less matter between wifi device (AP = Access Point) and where you consume wifi (wifi client).
  

• Internet connection should be centrally located (or to one side if you're going to need multiple wifi AP's).
  

• Avoid wifi usage whenever possible (especially anything that regularly plays a video stream - Smart TV, Apple TV, Roku, Fire TV etc). Only one thing can talk on wifi at a time per channel/frequency range.
  

• Use wifi signal strength to gauge number of wifi AP. 2 bars is the lowest reliable connection. The stronger the signal, the faster the wifi data transfer speed.
  

• Old wifi devices usually slow down the entire wifi network.

• Test and make sure you have a wifi problem and not an internet problem using test software (on a wireless PC preferably) and monitor.
  

Step Two: If that doesn't fix it we'll be going with multiple wifi AP's

If you have ethernet backhaul between multiple wireless AP's (the thing transmitting the wifi internet access):

• Unifi AP AC Lite (no local media aka video devices)
  
• Unifi AP AC Pro (local media video devices) You will only use the speed of the pro if you have a local Video server or transfer lots of data locally via wifi. Internet video is usually limited on speed by your ISP
  
• These require a server that I run to get setup and work long term.
  

If you don't have ethernet backhaul you need a modern mesh network

Here's a good analysis of the mesh options, and you'll get lots of features like restricting devices/parental features.

https://thewirecutter.com/reviews/best-wi-fi-mesh-networking-kits/