I also have the same problem, I think it is due to the fact that the printer searches for networks only when switched on. In fact, if the router is not already operational, the printer does not connect to the wifi network. A clarification or firmware update from Xerox would be needed.
I found this VIDEO for setting up the WiFi on your 6515 device that may be helpful.
I'm new user of an WorkCenter 6515 and try to configure WiFi access.
I see all others SSID's but mine. Tried to configure mannually but not option WPA2 Personal / AES
All other devices printers/laptops detecting my SSID automatically. All other devices (including printers) connected to WiFi via WPA2 Personal / AES
Any clue please how to connect ?