Step 1: Open the web browser and type the IP address of the firewall into the address bar (default is 192.168.1.1). Press Enter.
Step 2: Click on the plus sign next to interfaces and select Ethernet.
Step 3: Click on the WAN interface.
Step 4: Click on the Advanced Tab and Change the MTU to desired amount.
Step 5: Click on the Configuration tab and select Save and Activate from the dropdown menu. Click on OK to save and activate your changes.