Hello @waltshan
Getting WiFi coverage in outbuildings like this can be challenging. The Nest WiFi mesh interconnect runs over 5GHz only, and while that provides high performance at short distances, it doesn't work well at longer ranges. I suspect the only WiFi coverage you're getting out there now is in the 2.4GHz band, which is lower performance at short ranges, but continues to work at longer ranges. In short, it's unlikely any of the Nest WiFi units would work well in that garage. My advice for this situation is usually to try to run an Ethernet cable from the main unit (the Nest WiFi Router that is connected to your internet service) out to the garage, and then connect either another Nest WiFi Router unit (or a less expensive Google WiFi unit) to it. That way, the remote access point is talking to your home network via Ethernet instead of a wireless connection, but it can provide WiFi to clients in that area. It's definitely more work, but the result should perform quite well. If you decide to pursue this approach, I can provide more detailed advice.
If you really can't run an Ethernet cable out there, then you may be able to use (relatively expensive) directional WiFi bridges intended for long range operation between buildings. They would need to be permanently mounted on both sides, but at short ranges would probably work from inside each building. That's also going to be a fair amount of work (and expense).
