Run an ethernet cable from a LAN port on the Optimum device to the WAN port (the one closest to the power cable) on the Time Capsule. Then use Airport Utility to put the Time Capsule in "
bridge mode". Then use the Time Capsule for all your wired and wireless connections.
Once this is setup you will probably need to unplug the Optimum device then power it back up so it recognizes the Time Capsule.