This is related to my question at https://www.linux.org/threads/help-with-modemmanager-and-modem-on-uart-port.55625/#post-260222, but is different.
I have a SARA-R52M10 cellular+GNSS modem connected to Raspberry Pi CM4 UART3. The RPi is running Ubuntu 24.04. I have a ppp interface working fine...