Both the AP and the client work as expected. The connected TP-LINK N300 travel router also operates as expected. 

In a nutshell the complete system works. 

Client is near a road so I did not broadcast the SSID. I also changed the clients IP on installation. 

That was about 5 years ago and have lost the notes I made. 

Now I want to ping the client but I don't know what IP to enter in the AP. 

I don't know what IP to enter into the browser to open the login window. 

I have all N300, CPE-210 AP, and CPE-210 Client login data. (username & passwords) 

I cannot access the clients login window because I am missing  the IP and SSID of the client. 

The question then is how can I get the clients IP from either the N300 or the CPE-210 AP?

Physically getting at the client is very risky. 

Thank you very much for any advice. 

DrDan