A modem converts the signal your internet provider delivers into a connection a router can use. Cable, DSL and cellular services need one; fiber terminates on an optical network terminal instead. Confusing the modem with the router is the usual reason a site cannot reach its cameras remotely.
The modem is the boundary between the provider network and the building. Everything about remote viewing depends on what that boundary allows.
Modem, router, switch
The modem handles the provider link, the router shares it and applies firewall rules, and the switch connects local devices. Many provider boxes do all three in one case.
Where it fits with security
Bridge the modem and use your own router when you need predictable port rules and remote access. Double translation from two routers breaks port forwarding.
Related:Commercial securityGlossary: Video management system