Question about TP-Link Archer T9UH: AC1900 USB 3.0 adapter

Well, not at all. No WLAN device is capable of much more than 600-800 Mbit.
In your case, reduce the distance to the transmitter, no walls in between.
5Ghz WLAN is sensitive to these things and the counterpart should also be able to handle AC WLAN
with good performance.

This is also the case for me: 251Mbit/s because 2 walls and steel frames at a distance of approx. 6m.

Steel and concrete possible away, if possible.

How can you achieve the promised 1300Mbit/s?