It’s not a modem - it’s just a bare module
sierrawireless.com/en/produc … L808x.aspx
With a modem, all the hardware design is done for you; but you have to do all your own hardware design for a module like this.
So, the question is: is your hardware design correct?
In particular, is your power supply design correct 
See: FAQ forum/wiki?
And: Problems running Tlemetry samle .CREG=0,2 - #6 by nicool67
and many more…
Do you have good UK-Vodafone coverage where you are?
What does AT+COPS=? tell you?
Is your SIM valid?