Thank you for replying. Initially I suspected a router problem too, but it's not it because:
- the problem occured also with 2 different computers connected directly to the modem, without the router;
- the configuration worked OK for over a month and the problems started only after Ziggo did some "configuration" to the modem.
And yes, just to be sure, I updated too the router's firmware (with both files).
So far all the clues, including the regularity of the disconnects (about 15 minutes), seem to be pointing to a firmware or configuration problem in the modem.