PPPoE MTU

Hi all, I am using FortiGate 50A with 2.50 MR9. This FortiGate is using PPPoE connection. I found that MTU didn' t change even I set to override default MTU value. If ISP use MTU value 1492 and I set to use 1400, the result MTU is still 1492 after connected. So, how can I enforce FortiGate to use my own MTU setting? Thank you Martin