So, Last weekend I noticed that some DLCs were suddenly missing from my account and I wrote a ticket to the support. It's been several days already and I still haven't got any response and honestly, I'm not sure what should I do. Is there some way to contact them more directly?

It would be best to wait. Your ticket will more-than-likely be reviewed within the coming days.

GOG employs a rather small, over-worked support team (which was not the case until a number of years ago), and this frequently causes delays in responding to user queries, as they are constantly inundated with new requests for assistance.

While logged into a web browser, open the following links. Let the first one load fully in a new tab, then load the next.
https://www.gog.com/account/refresh
https://gog.com/user/refresh

If that doesn't help, there's nothing to do but wait for a reply from support. A week is a fast response time around here. Don't bother trying to message staff via chat for support issues.

Edit: forgot the links.

Did you initially -- upon first opening the ticket -- get an automated response from GOG Support?

If yes, this would seem just to be that GOG Support is swamped.

If no, it's unlikely your Support Ticket was successfully opened. This is rare, but it does happen from time-to-time. In this case I would open a new ticket.

Where did you buy those DLC?
If you didn't buy them here but bought a key at a key site, you might have bought a key that was revoked because it had been obtained fraudulently by the seller.

I bought them directly from GOG, so no, there's no chance of that (Not to mention that I bought them several months ago so even it that was indeed the case, I would have still been extremely odd)