Some time ago I installed B&W/HoS DLC's and all worked fine. However, as I wanted to install the game on another device, which I now use instead of the former one, (same GOG account) the installation won't proceed after a message saying I need 1.2 patch. Can't continue the main game without DLC's installed. How can I solve this (I'm no good with computers)?

If you have both DLC's and base game on GOG you should have access to GOTY version of the game as well. It has all of the expansions in a single install package. I don't know how the installers of the non-GOTY version looks, but if you insist on running it, it's a good idea to verify the game files through Galaxy. If you have a wrong version of the game, it should update it to the most recent one. There's also a possibility that some files are corrupted. Verifying them will fix this problem as well. Also make sure that you install the game and the expansions in the proper order. Start with the base game (update it if its version isn't 1.32), any bonus DLC's, then Hearts of Stone and finally Blood and Wine. You can also download and install the whole game from start with Galaxy. This should also solve your installation problem. You don't have to use the client afterwards if you don't want to, but you'll have to do a shortcut to avoid it launching on game startup.