Comment 2 for bug 1799795

Also, note that MTU is a read-only property in MM's bearer:

https://www.freedesktop.org/software/ModemManager/api/latest/gdbus-org.freedesktop.ModemManager1.Bearer.html

I think that the property is only known to MM for QMI/MBIM. In principle, the MTU should be a good one as it is negotiated, otherwise something is going wrong somewhere.