Hi friend.
To change the MTU of ONT you must change both the MTU values of WAN and LAN interfaces.
To change the WAN interface using the U2000 General VAS profile:
Step 1: Open General VAS profile
Step 2: Select the desired general VAS profile, right-click and select modify.
Step 3: In the next screen don't modify anything and just click next.
Step 4: Click modify.
Step 5: Finally MTU value will appear here, just select the interface that you want to edit the MTU and go through the submenu.
To change LAN interface using U2000 General VAS profile:
Export the VAS profile, and add the MTU="1500" for example, and then import to U2000.
I hope I can help you.