Depends on your sub’s content. If your sub is image/video based, then try to crosspost posts in your sub to other related subs/mention your sub in the comment section where it’s appropriate.
If your sub is text/info based. It’s harder to get followers. So here is a trick. First make sure your sub has good content. Next do a search site:reddit.com for queries related to your content. Help people, give them the info they want, and mention your sub in comment section in any post showed up.
You are not going to get subscribers as fast as the 1st type but here you have targeted users.