My E7270 came with a DW5811e WWAN-module. However, it seems Debian Stretch was not able to recognize it directly.
mmcli -L did not show any modems.
I did find /dev/cdc-wdm0 and a mention about it in dmesg however, as well as the error message:
ModemManager[2443]: proxy configuration failed: closed
ModemManager[2443]: <info> Creating modem with plugin 'Dell' and '2' ports
ModemManager[2443]: <warn> Could not grab port (usbmisc/cdc-wdm0): 'Cannot add port 'usbmisc/cdc-wdm0', unsupported'
ModemManager[2443]: <warn> Couldn't create modem for device at '/sys/devices/pci0000:00/0000:00:14.0/usb2/2-5': Failed to find primary AT port
After a bit of searching, I found this repo where the owner has a 5809e WWAN-module. Grabbing his rules.d-script and modifying it with my vendor and device id made NetworkManager find my card after a reboot, and now I'm able to connect.
Put this file in /etc/udev/rules.d/ and it should work after a reboot.
83 Mbps down, 46 Mbps up, 18 ms latency