The DHCP server in the modem firmware is similar to autoconnect. It is a feature designed to let the host ignore the fact that this is a modem, and just treat it as any unmanaged ethernet link. That’s nice if you don’t want to manage the mobile network connection.
I recommend letting the host manage the IP configuration. That is: Use QMI or MBIM to retrieve the connection information from the modem and configure the IP interface on the host. based on this. No DHCP.
I believe this is what NM+MM will do.