Deselect the checkbox before Auto Gateway, and then you will be able to select a WAN Interface from the drop-down list as the system default gateway.
The Auto Gateway checkbox is selected by default.
1) If changing the Automatic Assigned Default Gateway from unselected to selected, you have to reboot the Router to get the automatically assigned default gateway.
2) Default Gateway IP address should be specified since MER Interface is selected when you select the Enable automatic Assigned Default Gateway check box.
Please indicate the source reference, the original address: