Ideal Wan MTU settings: Peplink 30

I am using 2 Wan ports to connect 2 DSL modems in bridge mode, MTU setting 1492 for these two ports.
The third Wan port is connected to a data Lan network.
What is the ideal Mtu for this third port, should it be 1500, or 1492?



Normally will be 1500. Anyway you can set to Auto if you are not sure.

Thank you for your response.