In some instances it might make sense to utilize Modem Manager under libreCMC. This is a more full feature modem management app that is typically used on the GNU/Linux desktop. As routers usually have significantly less storage it's not the default.
These directions below assume you have already got a 4G modem configuration setup per our 4G instructions here. The below directions have been vetted under libreCMC v6.1, but please note that one element was left out of the libreCMC v6.1 build and repository. It was the luci-proto-modemmanager package. You'll need to contact support for the package or hope that by the time you get to following these directions the package has gotten into the libreCMC 6.x repository. The directions below assume it is in the repository.
To utilize the Modem Manager you need to install the following packages:
kmod-usb-serial kmod-usb-net kmod-usb-serial-wwan kmod-usb-serial-option kmod-usb-net-qmi-wwan kmod-usb-net-cdc-mbim modemmanager
The following package may or may not have been built for libreCMC and included in the repository, but will hopefully be included in the future. The lead developer was kind enough build the package manually for us and we tested this against libreCMC v6.1, and it worked as expected:
luci-proto-modemmanager
You will probably need to restart, go to System > Reboot and click the Perform reboot button.
In the libreCMC web user interface go Network > Interfaces and click the Delete button next to the wwan0 interface.
Then go click the Add new interface... button.
In the name box enter wwan0.
In the protocol box drop down select ModemManager and then click the Create interface button.
Enter the APN for your cellular provider in the APN field and then click the Firewall Settings tab.
In the Create / Assign firewall-zone drop down select WAN.
Click the Save button and then the Save & Apply button.
You will probably need to restart one last time, go to System > Reboot and click the Perform reboot button.
When the router comes back up you should have internet access.