A qmi_wwan MTU setting issue after switching raw_ip mode has been fixed with commit “net: usb: qmi_wwan: restore mtu min/max values after raw_ip switch“, recently integrated into master. The patch will likely show in v5.6-rc4.
The patch fixes a bug that prevented the setting of MTU values > 1500 to the network interface after a raw_ip switch (e.g. writing first ‘Y’ to file /sys/bus/usb/devices//net//qmi/raw_ip, then ‘N’).
The actual check is done by dev_validate_mtu that, after a raw-ip switch, is failing due to a max_mtu value different than the one set initially by usbnet. More details can be found on the commit body.
Check other qmi_wwan related fixes in qmap Linux driver fixes and driver history.