Maybe this is of help to others using the 3G usb modem function of the DSL-2750U:
I discovered that my ZTE K3571 3G stick (perhaps also others) only works on my DSL-2750U after a warm reboot of the dsl-modem, otherwise the webinterface shows as 3g modem status "NO USB CARD". So that means; Insert stick and from the web interface -> maintenance, reboot. After that the 3g stick works fine.
I think this is because after inserting the stick, the ZTE device presents its built in CD-ROM drive to the DSL-2750U which messes up the dsl modem 3G configuration.