Hello,
RUT360 does not support Port Based VLAN which is required for setting up LAN as WAN.
You can, however, convert the WAN port to a LAN port. You can find information on how to achieve that here: https://wiki.teltonika-networks.com/view/Setting_up_WAN_as_LAN
Make sure that your firmware is RUT36X_R_00.07.02 or newer as this is the required firmware version for this function.
Best regards,
Einius