Synology Assistant - Can't find my NAS after restart

[Rusty]

Yes, you are right. But the big point for buying this NAS was that it works as a USB print server as well. And it works fine with my laptop. So I don't understand what I am doing wrong.

Considering that you are a fresh user, IMHO it would be wise to get away from this setup you are using right now. Synology is killing support for a lot of DAS devices including printers, so it will be only a matter of time when this will stop to work altogether.

Either get a USB print server or a network MFP if this is something that you and your family will use most of the time.