You have 2 devices hooked to the coax cable ?
Generally unless you pay for multiple modems they will only allow you to have 1 device. You must register that device and you can't just change it without calling them and telling them. This uses the mac address of the modem to see who is authorized to be on their network but this may of may not be the same mac that the ip is assigned to.
With the surfbaord device that is only a modem the ip is assigned to the first device that is activated after you power cycle the modem. If you have your asus router then it should get a IP. The arris device is actually a router and it gets the IP itself. I don't know if you can use the trick of changing the mac on that device. The asus will never get a actual address if you plug it into the modem/router combo unless you make it into a modem only.
Since you have 2 modem devices your situation is hard to tell. Most times when people are trying what you are they can't get the ip to change but they at least get the same ip back over and over. You situation sounds like the asus is not getting a wan ip from the modem. You could plug you pc directly in and see if that happens to get a ip.