windows server 2025 pppoe mtu can not set to 1492 and ClampMss problem

Problem 1: Cannot change PPPoE MTU to 1492

The PPPoE interface ("寬頻連線") always shows MTU 1480.

Problem 2: LAN clients must manually set MTU to 1480

All devices connected to the LAN (including phones via Wi-Fi AP) that use the default MTU 1500 cannot open most websites.

Only after I manually change the client’s MTU to 1480 can they browse normally.

I tried enabling ClampMss and Forwarding on the PPPoE interface,is not work.