Go to Start/Run, then type telnet 192.168.0.1 and click OK button. Now enter your password (the original is 1234). You will get a menu.
Go to menu 24.8 (type 24 Enter, 8 Enter). This brings command line interface. At command prompt, type the following command followed by Enter:
ip adj wanif0 1452
This will increase the current MSS limit to 1452, hence the tweak test will report Max Packet Size as 1492. The catch is that every time your PPPoE disconnects and reconnects it will revert back to MSS limit of 1400 (Max packet size of 1440) and needs to be entered after PPPoE connects again. This command does not work before PPPoE connects.
Update: With Netgear firmware version 3.25 and later you can set the PPPoE MSS limit in a sticky way by using the following command (again via 24.8):
ip adjmss 1452
This will keep PPPoE MSS limit at 1452 between disconnects. If you would like to make it boot-safe as well, you must add the above command to the end of autoexec.net file within the router. To view autoexec.net file, you can type sys view autoexec.net command. To edit, type sys edit autoexec.net. The editor is a very simple line editor (like ed in Unix or edlin in early DOS). Editor commands are provided on the screen when you enter the command line editor.
Please note that the ip adjmss 1452 command is not effective until current PPPoE connection is dropped and reconnected. In other words, it is only applicable for future PPPoE connections. Log off, then reconnect before the setting will "take."