Then I used NOIP for years without any real issues, apart from the 30 day confirmation checks becoming a pain. I used their free option with no issues.
Asus routers have a free DDNS service that comes with. They also support OpenVPN, so if you talk your clients into a new router, you can set up both, and they can have a secure interface.
Ask if they really want their videos on the internet, or worse, open a port with port forwarding that lets bad guys into their home network.