D-Link DIR-859 - Why can’t I connect to certain sites or send and receive e-mails when connecting through my router?

86D-Link DIR-859 User Manual
Section 6 - Troubleshooting
You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented
packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the
proper value, the actual MTU size would be 1480, which is the optimum for the network we’re working with (1452+28=1480).
Once you nd your MTU, you can now congure your router with the proper MTU size.
To change the MTU rate on your router follow the steps below:
• Open your browser, enter the IP address of your router (192.168.0.1) and click OK.
• Enter your username (admin) and password (blank by default). Click OK to enter the web conguration page
for the device.
• Click on Setup and then click Manual Congure.
• To change the MTU, enter the number in the MTU eld and click Save Settings to save your settings.
• Test your e-mail. If changing the MTU does not resolve the problem, continue changing the MTU in increments
of ten.