As mentioned here, luci-proto-qmi is missing the option ‘pdptype’ in the GUI. Without it, it isn’t possible to configure the IP standard the WWAN-card has to use.
The error itself is also present in the /etc/config/network file, because as soon as you add the option to the QMI interface manually, the WWAN connection doesn’t work anymore. Right now, only IPv4 is possible.
A patch for the first problem is already available upstream, but I’m not sure, if it’s the final solution or if there are some missing bits left.