From the menu bar, select
▹ System Preferences ▹ Network
If the Network preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Then click the Advanced button and select the Hardware tab. Select
Configure: Manually
and
MTU: Custom
Enter an MTU value of 1280.
Click OK and then Apply. Test.
You must apply the change before it takes effect.
If the issue is resolved, experiment with higher MTU values until you find the highest one that works. The higher the MTU, the better the network interface will perform.
If changing the MTU has no effect, restore the Configure setting to Automatically.