the major services require 20-25 Mb/s for 4k, and around 5 Mb/s for 1080
Since you're running on a 1080 TV, pretty much any wifi is going to be faster than you need.
if you are having problems with buffering, try moving the appleTV to a different location, Mine is stuck in the wall mount behind my TV, and there are certain places it doesn't work as well. Interference from the electronics in the TV, or being blocked by metal in the mount can both cause issues, Sometimes moving it a few inches one way or the other can clear it up.
Speed test measures your internet speed which may or may not be the same as your intranet speed. For that you need to run something like Network Speed Tester, a server on your or other device and the client on the Apple TV, or visa versa to test your internal network speed.
Unless Netflix, Apple, and the other streaming services have set up a server farm in the garage, for this application, Speedtest is fine.
If you are streaming from something on your network (like plex), chances are it will have similar or lower bandwidth needs compared to one of those services.
For devices besides the appleTV, then the internal test is preferred, but on the aTV, all that really matters is that the stream downloads in a shorter time than it takes you to play the media back. If it takes you 2 hours to watch a movie, it doesn't matter if the stream takes 5 minutes or 90 minutes to be received by the aTV.