If you decide not to use a fixed IP SIM card and instead want to rely on Dynamic DNS then we have prepared a support video showing how to do this.
As far as we are aware the only UK mobile network that will provide a public IP address is 3mobile and you need to use the APN of 3internet and connect using 3G – it is not guaranteed that 3mobile will allocate your connection with a public IP address and it may be stopped at any time. DDNS will not work unless your SIM provider / mobile network provides your connection with a public IP address.