The Problem was deeper. The BE800 doesn't support Multinet. So i have to buy a Gateway-Router, which i did, and this solved my problem.

I have a TP-Link Archer BE800 (BE19000) with the network address 192.168.0.0 to which a TP-Link SG3210X-M2 is connected. This managed switch forms its own subnet (192.168.10.x). Computers in this...