@ladtract 

Even if N300 is compatible with Deco, it is unlikely you'll achieve what you want to.

N300 is WiFi range extender. The concept of range extender is to pick good WiFi signal and retransmit it. If WiFi signal is already bad where you plan to place range extender, it won't work well.

If you check marketing pictures of range extenders from different brands, you'll notice they are placed about in the middle between source of WiFi signal (that would be Deco M5 in your case) and WiFi client (laptop, smartphone in garage, in your case).

In your case, that would mean placing range extender in the middle between house and garage, I doubt you can do that.

To emphasize: that is applicable to any range extender of any brand.

Options that you have:

If you have TV cable going from house to garage, or garage sits on circuit breaker coming from your house, you may be able to use these to extend network to garage. With additonal relevant equipment, of course.

If not, you may have to look for directional WiFi signal boosters. For example, check if something like this can work for you: Dedicated Long-Range Outdoor Wireless Networking Solution