Did you assign the Linksys a static IP address in the Linksys WAN interface?
If so, it won't appear in the DHCP connection list, but will appear on the main status page and the network status page.
If you didn't assign the Linksys a static IP address in its WAN interface and it is getting its WAN IP address via DHCP, then it should reappear in the DHCP connection list the next time it renews its DHCP lease with the Actiontec, which could be as long as 12 hours since the Actiontec's LAN DHCP lease interval is 24 hours. You can force the Linksys to request a new DHCP lease by rebooting the Linksys.
BTW, the Actiontec is a MOCA capable router. It is not a modem.