Cannot roam between WiFi Router & WiFi Access Point on same network

I have a seemingly simple desire: to achieve acceptable wifi coverage of my entire house and the ability to use my wireless devices in multiple locations within that house. To achieve this aim I have tried a variety of hardware combinations over the last two months but am now at the point where I'm almost ready to abandon the project.

In my current setup I have:

• Downstairs: A new & working Wireless Router (TP-Link TD-W8961ND) from which I can connect to (1) the internet, and (2) the internal network devices (NAS, printer, and network shares). Broadcasting on channel #1. DHCP = On.
• Upstairs: A new & working Access Point (TP-Link TL-WA801ND). The AP is connected by a physical cable to the downstairs router. The AP is in "Access Point Mode" with the same SSID and security settings as the router. From the AP I can also connect to (1) the internet, and (2) the internal network devices (NAS, printer, and network shares). Broadcasting on channel #11. DHCP = Off.

Clearly therefore, parts of my setup are correct and complete. However, I cannot roam between these two devices. Once a device is connected to one of these devices it cannot connect to the other. When downstairs, if I connect any device (iPad, iPhone, Win7 ThinkPad, WinXP Netbook) to the router's Wifi network, it connects to the router as expected and is fine. When upstairs, it connects to the second router but cannot get to the internet (it gets an IP address beginning with 169.). It reports a "Strong" connection but simply cannot establish a useful connection with the world outside the house. The same is true if I take a machine that is successfully connected to the upstairs network and move downstairs.

Have you successfully setup a wifi network with a TP-Link router and a TP-Link Access Point so that roaming works? If so, what's the secret?! Should I use the "Bridge with AP" mode for the AP?