I now switched on the amplifier
B.1.8.2 Process for Enabling and Activating Power Amplifier/Low-Noise Amplifier
as instructed by the documentation.
The RSSI level has increased quite a bit, unclear though, how it should bridge 400m, RSSI drops rather rapidly with short distances.
Any further ideas ?
Please take a look at this article which give an explanation about how to obtaining the 400-m range.
thanks zhxh, this is very helpful. Cheers Stefan