Hi Fabian,
Please make sure that the USB configuration is correct, https://source.sierrawireless.com/resources/airprime/hardware_specs_user_guides/airprime_hl8548_and_hl8548-g_product_technical_specification/#sthash.KD7jK7o2.dpbs
You can refer this topic to upgrade your device HL8548 firmware update
Thanks,