I attempted to set it to the AP to AP client mode but it requires the MAC address of the remote AP which I am assuming is the router. When I attempt to enter the MAC address, it gives an error saying that the MAC address is not valid. The search feature of the AP setup utility does not find the router either. Hmmm.

Here's what the manual has to say:

AP (Access Point) Client - When set to Access Point Client mode, the Access Point Client is able to talk to one
remote access point within its range. This mode allows the Access Point Client to act as a client of a remote
access point. The Access Point Client cannot communicate directly with any wireless clients. A separate network attached to the Access Point Client can then be wirelessly bridged to the remote access point.

Enter the required LAN MAC address of the remote access point in the Remote AP MAC Address field.
To select an available access point, click the Site Survey button and choose from the access points listed by
clicking on the radio button for the appropriate access point and clicking the close button. If you do not see an
access point listed, click the Refresh button and another survey will be performed.