Dave has noticed that when he powers up his MacBook away from his Airport, it won't get better when he gets closer to it. So he bought an Airport Express to relay the data speed. Leo says that's a good solution as it acts as a repeater. But he'll have to be sure it's set up properly.
Dave should launch Airport Utility and set up the Airport Express by adding it to the existing network. The newer ones are dual band 802.11AC. Also, remember Wi-Fi is supposed to be line of sight and the higher the frequency, the more the signal will bounce around. If he has metal in his walls, that could also impede the signal.
Leo also says that the network is likely congested with interference from other routers in the neighborhood. That's why a dual band router with 5GHz can help clear the way. Some routers can grab all the available bandwidth around and jam everyone else, so that could be part of the problem.