FX30s Change MTU size on rmnet0 connected

We want to change the mtu when the rmnet0 interface comes up, but via ifup / ifdown scripts does not work because rmnet0 does not seem to be managed by networking.

What are the possibilities to adjust this when a connection has been established? (so triggered by an event)

Running the stock Yocto build from URL
Release 14.0 incl. Legato