Hey @pokkel, 

Sorry for the delayed response,  if adapter fails to pick uyp 5Ghz wifi signal, you could try to change the 5Ghz channel to 36/40/44/48  as the guide here:

 Why cannot the adapter detect the 5G Wi-Fi of my router?
https://www.tp-link.com/support/faq/2989/

For unstable speed issue, it is recommended to download wifi analyzer app and select a less crowded 2.4Ghz channel & 5Ghz channel for main router, and see if speed could be more stable. 

May I know does this happen on 2G or 5Ghz wifi band, at the location of T5E, please run speed on a different device, what is the speed,does that drops or be stable? 

Kindly note that when multiple devices share the total bandwidth from router and device is on different network activities, the real time speed could change from time to time.

Please leave only 1 devices running when device do speed test and turn off unnecessary network activities.