What manufacturer and model-number of router is supplying your home WiFi network?
How many walls between your router and your garage? What's in the walls? Metal posts? Wood?
In your garage, do you have a wall-rack with lots of metal tools mounted on the rack?
Connect your mobile-phone to your WiFi router, while standing beside the router. How many "bars" of reception? Move one room closer to your garage. How many "bars"? Keep moving & measuring.
Move your vehicle out of the garage, so that the metal in it does not interfere with the signal-strength.