Router R6400 won't appear in Network List

[garfield7000]

Hi everyone,

I'm currently trying to use a used R6400 router with a Spectrum modem. I successfully hooked the router up the modem (all lights white on router; modem says it is online), but the router will not appear in the network list. I'm not sure what's wrong, and the previous owner of the router refuses to tell me if he changed the name of the network. Could it be a compatibility issue with my ISP? Wouldn't that mean the router would flash amber? 

Thanks.