Try powering off your modem and leaving it off for 10 mins, then try again. Cable modems can be picky if you move them to a new router, they might be unable to obtain a new DHCP lease from the ISP unless you keep the modem offline for at least a few minutes.
You could also try cloning the old router's WAN MAC address.
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.