I'm back after not having internet for some 150+ hours, been in contact with my ISP and they had no idea what was wrong.
So finally I decided to fiddle with my cable modem by myself and realized that I constantly got a time sync error when the modem tried to connect. So I fiddeled around in the settings and found that "fetch time from server" was disabeled BY DEFAULT!!. Turned that on and imediatly the modem seemed to be able to connect
FFS my ISP even has it's own firmware in the cable modem how the hell can they have a function that apparently (I don't know for sure since I might just have enabeled the setting when the ISP "fixed" the problem by shear luck) is cirtical for my broadbands function, being default as off???
But I'll wait and see and hope it doesn't go down again