Surveillance CMS for a main and sub office.

Hi, I am a student and I have a thesis for my client which is setting up a cms for their main office. Their business has 3 branches. The main one and 2 other branches which are factories. What I would like to do is have a CMS in the main office for their convenience. The things which should be in the view are the cameras of the main office and the 2 other branches. In the other 2 branches, we don't really need to do any business, because the only thing they allow us to meddle with is the surveillance footage, all we need to do is have that surveillance footage directly to our main office that's it. We currently have a Synology NAS(Network Attached Storage) which we will use to store the surveillance footage. It has a Surveillance Station feature which also acts as a CMS but it can only handle the local network(network for the main office). We couldn't connect the IP cameras from the sub branches to the surveillance station which is our main problem. Also, what is the best CMS software for multiple networks we can use if don't manage to do it in our Synology Surveillance Station? Thanks a lot!