Hi June and beady,
a very "Simplest method to register .." is described in:
https://www.mobileread.com/forums/showthread.php?t=87686
I also live in a country without whispernet, so I tried this and it works just fine. Now I can make collections on my Kindle DX v2.5.2, but it would also be nice to have at least a cumbersome way in calibre, because I also have a large batch of books.