@Menna 

The purpose of a router is to create a local network that then translates through a single source and out to the internet.  The router does not report the number of client devices to the ISP nor can the ISP see any of your clients,  NAT is how this whole process take place.  It tranlates data from your internal LAN (local area network) to the WAN (wide area network).  Their support mislead you.   To troubleshoot the issue properly we would need to know the make and model of your modem and your service type.  i.E. Fiber, cable, DSL, etc.