So, I installed the new version and suddenly, wonders happend: I have propper WiFi!
FINALY after all these years my CC Pandora has more than a few kb/s download speed, actually I have full bandwich of my humble 2K DSL. ^^
So it was in fact a software issue and not faulty Hardware (antena) or a general router incompatibility. Still mysterious because other CC users never had these issues with the old Firmwares (some had though). No clue what notaz ther did in detail but it was the fix for my WiFi suffering so many thanks again.