sorry if this is already mentioned but you can actually test how much your MTU must be reduce by dong this ping command from the PC
1) Determine the WAN gateway
2) From a command prompt on a PC, type PING -f -l (MTU)(WAN gateway) . Start with an MTU size of 1,454.
3) If this fails with an error message indicating that it must be fragmented, decrease the MTU size and try again. Repeat this until the ping command succeeds
