To connect a router to an optical network terminal (ONT), ensure the ONT is properly installed and connected to the fiber optic cable. Next, use an Ethernet cable to connect the LAN port of the ONT to the WAN or internet port of the router before powering on both the ONT and the router. Configure the router settings according to the service provider's instructions, which may involve entering specific IP addresses, subnet masks, and gateway information. Once configured, the router should be able to distribute the internet connection from the ONT to various devices on the local network.