Follow the router's Quick Installation Guide. Or User Guide. Briefly, with everything off, connect modem to router's WAN (Internet, modem) port, one PC to router LAN port with ethernet cable. Optionally connect 2nd PC to router via ethernet. In order, plug in or turn on modem, router, PC(s).
If no internet access, probably means your ISP keeps track of MAC address of the device connected to the router. Access the router through your browser and clone that MAC address of the PC that has internet access directly through the modem.
Configure the router's wireless, and optionally connect either or both PCs via wireless.