Typically you’d have the VLAN tag set on the modem (assuming the ISP specified this) the router is on the “LAN” so I’d check that first. Setting DNS on both modem/router to 1.1.1.1 and 8.8.8.8 will rule out any DNS issues. Finally is the modem set to bridge mode? Can you connect a wired client to the Internet directly into the modem? ping and traceroute are your friend in this situation. ✅😎