Is there any way to change MTU size?

I'm using pppos to communicate with a SIM800L module.

All connections work except for RPC responses which get sent by the device but are never received by the mDash server, the SIM is a specialised IoT SIM which works by roaming on other operators.

I've been been advised to decrease the MTU size to 1400 but can't find a way to do it without changing the underlying lwip ppp library. Am I missing something?