The truth is that private servers shouldn't be advertising themselves with large ads, hiring YouTubers, streamers, and general advertising. We don't own the intellectual, artistic, or even technological property behind the servers. Many companies don't care about "small" private servers, as long as they know their place. Ragezone for example, its okay. Its a forum for developers, we know every game advertised here are private servers.
I've seen servers acting like they're the developers and owners of the IP. Thinking they have real rights over the game. So they hire (with contracts) streamers, create absurdly expensive ads... And some even go so far as to impersonate official servers, deceiving the player base. There are private servers that go to official channels to post about the pirated service, with the utmost audacity.
But I agree, as a server owner, that there arent ways to bring more players atm. I used to advertise on Facebook MMORPG groups and Ragezone only. But Ragezone is for developers, I dont think permanent players will join it by ragezone. As for Reddit, I never tried it. But in reality I dont think there is a good place to advertise in the fair way right now, or even for players to indicate and share opinions about servers in general.
A few years ago I used to go to XtremeTop100 to get to know private servers, but it seems abandoned and full of bots (there is a wow server that even get random servers names from different categories to get included there)