DDNS is Dynamic DNS, so rather than referencing the external IP address of your router, which could change if the router is rebooted or service goes down, you would use DDNS service (usually on router) which would update the IP address on whatever service you choose to host your DNS name. i.e noip.com is one such service but there are plenty of others out there.
Do you still have your old router? If so you could boot it back up and take a look around the settings to see if you can find any reference to DDNS and if so what details it has and service being used. At the same time you could also check what port forwards where setup previously for you CCTV cameras and again note detail.
Problem you now have, assuming you are using the new EE SH+ router is that they removed the DDNS service which is absurd . There has been plenty of discussion recently on this on this forum. It might come back reading between the lines. But this does not help you now. Are you using EE router or your own one?
As a temporary measure to at least prove you can view your cameras, you could go to https://whatismyipaddress.com/ and note your IPv4 address. You can then use that address within your app you use to view your cameras assuming you have correctly setup port forwarding on router. If you have not already modified what is in the app you use for viewing, note down what the current IP address/dns name is currently set.
