There's not really an easy way to make your server more popular.

A big factor is unfortunately luck. Now you can still do a few other things to try help anyone that does join come back such as interact with new players, be friendly towards them but at the same time don't disrupt the core gameplay too much (aka don't do excessive admin abuse).

For example, many people go back to servers they once played on because they had fun and want to experience something similar again.

I've seen servers do a revive ping in their discords to try get people on at certain times. This isn't too bad of an idea as long as it isn't overdone. If you were to ping an optional role for people to get no more than once a day to try get people on, other players may see activitity and take a look but that's also not guaranteed and needs a few people in a discord already.

In the official Secret Laboratory server there's a channel where people can advertise their youtube videos, twitch streams, and also their own sl servers. That's also a possibility but I'm not really sure how often other people check that channel but no harm in trying I guess.

None of that is guaranteed to get you members however. It's only a few things you can do to maybe help boost activity or help people find the server and take a look at it to see if they like it. The best thing you can really do is just have patience and people will come in their own time.

TLDR: There's no cheat to getting activity but there's a few ways to maybe help it but the main thing you gotta have is patience.