Moved to General Hardware.

I bought a set of the Google Wi-Fi pods and set them all up in-app to work as a mesh network. My entire house is covered and my 4K TV (the furthest end away from my hard-wired modem/router) still pulls down 4K content with ease.

I just use a basic Wifi extender from Netgear. It really depends on what you are trying to do though while outside. For me at the time my goal was just to get a signal at all. I can stream Netflix if I wanted but mostly it was about checking the internet at the time and that was 4 or 5 years ago. I think I spent $59 on it.
 
Guess the other question is what size is the back yard? If you live in mansion land and the back yard is the size of a football field the recommendation would be different then if you could throw a football and hit a house three doors down.

If security is the most important aspect it might be worth looking into Wifi 6 or AX and WAP 3.0 support. Right now it’s expensive for everything but by the end of the year that could change.

Just get a repeater since they're easiest to set up. It'll just piggyback off your existing wifi connection (so it'll still use WPA2, assuming that is what you are using for security) and provide a second SSID to connect to. Repeaters will halve your bandwidth on the second SSID however due to overhead. If you really need all the bandwidth you can get, you will have to look into setting up a WDS (most difficult as all routers/APs must support WDS with the same security settings) OR buying access points and connecting them via ethernet to your main router. WDS has the convenience of being completely wireless, however connecting access points over ethernet to the main router would be more reliable. Diagrams below:
 
WDS

 
 
 
 
 
Router with Access Points

Delirious
I guess I'm lucky.  my nighthawk router sends signals all over the house and garage and into the front yard to the curb.   about 3500 sq ft home.   could try a better router or the extenders mentioned. 

That AC66U router has a great range - If the software setting is set to Max Power & side antenna @ 45 degree angle
 
I've got one on the second floor that has a 2.4 range of over 300 feet

Two usual ways adopted are: 
1. Buy a high range router, which usually expensive. 
2. Use wifi repeaters or boosters.
 
Sometimes the best way to get better Wi-Fi signals is to just change your Wi-Fi channel. Here’s how.
Changing channels can make the difference
Wi-Fi works on two different frequencies and a number of channels. Finding the best Wi-Fi channel available to you can really help improve your connection.
 
Is one Wi-Fi frequency better than the other?
The two Wi-Fi frequencies are 2.4GHz, and the newer 5GHz.

• 2.4GHz has more range, but is more prone to interference from neighbours’ channels, which can slow your speed
• 5GHz can be faster but may not work on older modems.

Using scraps, you can supposedly extend your wifi signal over a 5 mile radius (over 70 square miles).
 

 
Or, like I said above, it's usually easy to convert that cheap router -- which is just laying in your closet -- into a wifi repeater.