The engineer at your company was on the right track (although "low quality" feels a little harsh ). Your Rachio controller can only connect to the 2.4gHz band of your wifi. This was actually by design, not a way of cutting corners. Since we aren't streaming Netflix on your controller, we don't need the speed of 5gHz. The 2.4 band is better at reaching farther distances, and is less affected by obstacles like concrete walls and storage cabinets. Considering controllers are typically in garages or basements, the 2.4gHz compatibility seemed like a better fit for our application.
With mesh routers, they switch devices between the two bands. Since Rachio cannot work with the 5gHz bands, they do create connectivity issues for controllers. And since it is seeming like mesh is the "wifi of the future", we will need to come up with a better solution in the future! In the meantime, I'm glad the tin foil worked for you!