Motorola MB8600 modem with Asus AC2900 wifi router. No internet on the router.

Anyone had experience pairing Motorola MB8600 modem with Asus AC2900 wifi router? When I am connected directly through the modem, everything is fine. The wifi router though can't get internet connection from the modem, saying "your ISP's DHCP does not function properly". Any ideas? The router setup was done through the wizard: Automatic IP, no password.