iMac (El-Capitan) - Trying to make Wi-Fi as a priority over Ethernet

I am trying to prioritise Wi-Fi over Ethernet cable and have changes settings to Wi-Fi under service order but when I do the speed test on Chrome the internet is used from the cable which is only 3 Mbps but on my wifi I get 20 Mbps. Both connections are on a different network.

Is there any way where I can use the internet service from Wi-Fi connection and local file transfer using from the Ethernet cable?

Solution 1:

1. On your Mac, choose Apple menu > System Preferences, then click Network.

2. Click the Action pop-up menu , then choose Set Service Order.

3. Drag services into the order you want.

4. Click OK, then click Apply to make the new settings active.