By default a router would reject all incoming connections. Your alternatives are:

• Configure the ISP router to forward a port.
• Configure the ISP router to set your own router or PC as DMZ, which means forwarding all connections.
• Configure the ISP router to work in bridge mode rather than router mode, meaning it will act as if it wasn’t there (but will still login to the ISP connection) and let your own router or PC handle things.
• If you cannot configure the ISP router in any way, only solution remains to replace it with your own router. Whether this will work depends on how it connects to the ISP (both the physical connection type and the login).