Comment 1 for bug 1799795

I would say that in general the cellular connection MTU is negotiated:

1. By pppd for modem that use serial connections
2. By ModemManager for MBIM/QMI modems

In case 1) (which is gemalto modem for rigado), you can actually control the MTU using NM, as NM launches pppd:

$ ip l show ppp0
6: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1430 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 3
    link/ppp
$ nmcli c modify pepe ppp.mtu 1000
$ nmcli c down pepe
$ nmcli c up pepe
$ ip l show ppp0
7: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1000 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 3
    link/ppp

For the Quectel modem, which uses QMI, it seems correct to me that you cannot set the MTU permanently in an easy way. But, it is quite surprising that you need to do this. QMI should negotiate this properly, and in fact I have never had problems with MTU sizes in cellular connections. Is this really a problem you are finding? Verizon connections worked fine for me when I tested the modem.