Data to pass between the Internet and your network, the information must travel through virtual ‘holes’ in the router’s software, called ports. Ports not only allow the information to pass in and out of your network, but they also direct the information to the correct devices. 

There are 65,535 ports on your router but no worries, they are not all permanently open. 

Secure ports are either opened manually by the owner of the network or by inbuilt protocols, such as UPnP and NAT-PMP. These protocols are designed to save users the effort of having to manually open ports – they give permission to the devices within the network to open ports on their own. 

What device do you have? A modem only device won’t have any settings for that so you’d look to having a halfway secure router. but its unknown what you actually are using.