i try but no luck....needs a dvb c usb key???
I must state I do not own a VU+ Duo but, I think you are changing the wrong thing:
DVB-C / DVB-C2 stands for "Digital Video Broadcasting - Cable" (hence DVB-S / S2 = Satellite & T / T2 = Terrestrial) and it is the DVB European consortium standard for the broadcast transmission of digital television over fibre cable networks.
This setting is nothing to do with C-Band (C-band transponders use the band of frequencies from 3.4 to 4.2 GHz GHz. LO frequency 5150MHz) Satellite reception.
Unless by C-Band you mean Cable in which case you are using the wrong terminology. As C-Band is usually associated with Satellite as far as Satellite forums are concerned.