just want to give you a feedback. The problem was a WLAN-Bridge which hadn't configured a DNS server nor a default gateway because it doesn't need it. It only bridges a LAN Client to another LAN via WLAN. And DHCP Server is a Firewall on the other LAN side.
Gave the AP-Client a DG and a DNS Server and after this also NSlookup on the Libreelec box was working. It seems that the WLAN-Bridge stops forwarding DNS-Traffic when it also has no DNS Server,.... I don't know why, but i can see DNS-traffic on LAN side of the Bridge and on the other side on the other LAN DNS traffic doesn't appear,....