The ALEOS user can not change the WAN MTU.
The decision on MTU size normally rests with the device that is creating the packet. Save for PAD mode packets, for which we do now have the ability to set the MTU, the gateways are not creating the packets. The host device is creating the packets, and we are simply passing them from the host device on to the cellular network. We do offer the ability through DHCP Options to tell the host device what its MTU should be, however. The onus for setting MTU is really on the device that is creating the packet. We take that responsibility for the PAD mode packets that we create, but other than that, the host device behind the gateway should be taking responsibility for the size of the packets it is creating and sending.