Hello and thank you for your help.
It seems, that all my "mobile problems" were just caused by the old router.
I have installed the new one and as much as I can tell, the mobile access is working now.
Still have to test it in my office and need answers from my brothers who have to test it too, but it looks fine.
One last question I have, related to the passwords:
When I enter my network with my laptop. How does the MBL see, who I am? Is the login from the WD browser interface stored somewhere? Do I always have to log in by using the browser interface?
What, if I now use a new laptop in my network. What folders from the MBL will I see there and if I don't see anything or only the public folders (what I hope, is the answer here), do I have to log in on the browser, so the network paths will be opened?