The CCTV software that uses the 3G will need to support DDNS and from what you say that is the case. You now need a DDNS provider... personally I recommend DynDNS which offer some of the best DNS solutions that are reliable and not to expensive. I recommend the pro package which in their own short description sounds exactly what you need:
Looking to access your computer, DVR, webcam or camera system remotely
without having to remember a confusing (and ever changing) IP address?
Try DynDNS Pro, a great option for those who need Dynamic DNS and
Remote Access capabilities without any bells, whistles or fireworks.
There are many other DDNS providers however and you should take a look here for free DDNS and other paid DDNS services. If the CCTV system is important I recommend a paid service as they have a habit of stop working on free based DDNS