The TL-WA901ND is positioned upstairs and connected to the switch via Ethernet.
I setup the TL-WA901ND to run in Universal Repeater

How is that possible, connected to a switch on UR mode, UR mode is wireless, thats the purpouse, just unplug it might need a reboot and will work.
UR mode has a 50% or more data rate penalty, plug Ethernet and use AP mode same SSID same password of your Network.
The only device that does zero handoff or roaming its the UNIFI from UBMT read here http://community.ubnt.com/t5/UniFi-Frequently-Asked-Questions/UniFi-What-is-Zero-Handoff/ta-p/412719

Hi, it might well be that I'm using the wrong mode then? My understanding was that in Universal Repeater mode, it was effectively using the LAN from the main router/AP to the part of the house with poor signal. The TL-WA901ND setup in Universal Repeater mode would then transmit the signal to that part of the house. Communication between wireless devices and the internet would not be entirely wireless, but rather would be wireless between the device and the TL-WA901ND, which then sends the signal over Ethernet cable back to the router which then connects to the internet via the cable modem. My understanding was that using the Ethernet cable rather to feed the TL-WA90ND rather than it connecting wirelessly to the main router would prevent the 50% throughput reduction associated with using a repeater in purely wireless mode. Network tests on the TL-WA901ND show pings of around 15ms and download speeds of around 60MBps, vs. 70Mbps on the main router's SSID, so not a major drop-off. If I could just get it to use a single SSID and automatically transfer between networks as devices move about the house, that would be perfect.