DIAGNOSIS This is almost always a classic "order of operations" issue during setup. The router is likely trying to pull an IP address from your modem before the modem is fully ready to hand one out.
FIX Let's do a full power-cycle sequence to get everything talking. 1) Unplug both your modem and the Atlas router from power. 2) Wait two full minutes—this lets your modem's connection fully drop at the ISP side. 3) Plug only your modem back in and wait until all its normal lights are solid (usually 1-2 minutes). 4) Now plug in your Linksys Atlas router. Wait another minute for it to boot. 5) Now try the setup process in the Linksys app again from the beginning. This sequence ensures the modem is fully online before the router asks for a connection.
IF THAT DIDN'T WORK Double-check the physical cable. Make sure the Ethernet cable from your modem is plugged into the correct port on the Atlas—it must go into the Internet port (it's usually a different color, like blue, and might have a little globe icon). Using one of the regular LAN ports by mistake will cause this exact error.
HEADS UP During setup, if you have an ISP combo modem/router, you need to put that device into "bridge mode" or "passthrough mode" before connecting the Atlas. Having two routers trying to do the same job will cause this failure. You'll need to log into your ISP's device settings to do that.