Sorry, but we don't know whether you are trying to "link" your AirPort Express to the Optimum router using a wireless connection.....or....a wired Ethernet cable connection, so we will need clarification on that, please.
Once you do have the "link" established again, what is it that you want the AirPort Express to do? In other words, what is your networking goal for the AirPort Express?
In order to establish the "link" that you ask about, it will likely be necessary to reset the AirPort Express back to factory default settings and then set it up again to work with the Optimum router. To do that we would need to know if you are still using Windows 7 to administer the AirPort Express, as you indicate in your profile or whether you plan to use a Mac or an iPhone or iPad to set up the AirPort Express.
Once we have a more clear understanding of how you want the Express to connect to your network, what it is that the Express will do, and what device you will be using to administer the Express, we will likely be ready to proceed.