2.4Ghz devices can affect your speed but ,Radio interference (2.4Ghz phones, etc) shouldn't have an effect on MTU.
It has to do with your connection type.
"Q: MTU: How high can/should mine be? (#652)
A: MTU (Maximum Transmission Unit) will vary with connection type. Cable and non-PPPoE (Point to Point Protocol Over Ethernet), can use up to 1500.
PPPoE connections (WinPoet, RASPPPOE, Enternet, etc.), can only use up to 1492. However, WinPoet and Enternet MTU must be set by hand in the registry. Others may use DRTCP to set MTU."