I have just upgraded the firmware of my TP-Link TL-WA860RE 300Mbps WiFi range extender to Version 3.15.8 Build 150820 Rel.40814n.
The most visible enhancement to this new firmware is the addition of Proxy Mode on top of the existing Universal Mode and WDS Mode of operation.
The Proxy Mode is said to have the best compatibility with routers and access points of all brands. If your range extender has problem working in Universal Mode and WDS Mode, you can try using this Proxy Mode. However, it may decrease the overall performance.
Note that you might lost all your existing settings in the TL-WA860RE after the firmware upgrade. Anyhow, there are not much settings for the device, so you can redo all the settings without taking up much of your time and effort.
You can click here to download the firmware for TP-Link TL-WA860RE version 3.15.8.
To perform the upgrade, unzip the downloaded file to get the wa860rev1_en_3_15_8_up_boot(150820).bin file.
Login to the WiFi range extender, and go to System Tools > Firmware Upgrade to perform the upgrade.
I strongly recommend you to connect to the device using a LAN cable instead of connecting through WiFi to perform the firmware upgrade.