Networking diagram:

Configuration Steps

Step1. Access the gateway's management webpage, go to Network > MWAN, and Click on the ADD button.

Step2. Set the Interface Type to VlLAN-Tag based WAN Port. then click the Main Interface to WAN and enter VLAN TAG.Enter the Username and Password given by your ISP. as shown in the following figure:

Repeat steps 1-2 above to add the remaining ADSL accounts. 

Note:  The VLAN TAG must be the same as the VLAN TAG created on the managed switch.

Finally, you can see the ADSL line that has been given the IP address.

Using only 2 ADSL lines as an example.

You can verify the effect of load balancing by speed measuring software.