Does the xbox connect to the the router at all?
Be sure you network factory reset the Xbox, turn OFF then back on.
Are the wireless modes the same being used on the AP and main host router? Just different channels?
Will the xbox connect to the wireless if you temporarily disable WiFi security?
Do you have the 815 set up as a wired AP mode using the LAN port in back to the main host routers LAN port in back right?
Maybe someone can review your router settings with you using Link>
teamviewer if your interested. Its safe and secure.