Make sure you have the router elevated up. They higher the better. Then make sure that you play with the antennae where it’s not just the 3 pointing up. Aim one horizontal and play with different directions. Best to also make sure nothing is blocking the signal. Then take you cell phone and download a Wi-Fi signal app and start walking towards your shop. See how far you get before the signal drops. Then mess with the antennas again and see if the signal doesn’t improve.
Make sure it is not set at 5gz. The 2.4 will have better range for his shop.