DG - Default gateway, which is the router in the house. As you connect to the wifi in the house, the routers IP and Mac address become your default gateway. So when you move into the garden, you connect the the same SSID, however this time its for another physical router, which a different IP address. As it cannot see its gateway, it wont send the traffic to the router in the garden. So you need to either mesh the two routers together so that you can seemlessly connect between the two, or put the garden router on another SSID (its ok to combine 2.4 and 5), set the router to DHCP so it picks up an IP address from the house router that it is connected to and away you go.