Thanks for your quick help! Problem was not the DNS, but a poor connection. When I tried near the access-point, I used another bridge (too lasy to rip the whole install from the wall) – and that one had a DNS-problem, so you were right.
Conclusion: Wifi has to be >= 75dbi
I solved the whole thing by daisy-chainig two PLC-networks….now it’s working like a charm!