How to change the MTU value of a non-WAN Interface on Gen 5

Here's the workaround:

1. Go to Network | Interfaces and click the configuration button of the interface.
2. Change the zone as "WAN" zone.
3. Go to the tab Advanced and set the "Interface MTU" to 1500.
4. Click OK.
5. Click again on the configuration button of the interface.
6. Re-assign the interface to its original zone.