You have to navigate to the comment or post that you banned them from, click the three little dots next to their comment/post for more actions, and there should be an option to unban
You may have to go to the modlog and click the direct link to their post/comment to find those options if they have no other posts or comments in your community.
There's no direct link from the modlog if someone's banned, apparently. I was able to find one by googling to find the post. The second ban I'd like to reverse, I removed the comment, and thus it can't be found.
If anyone has further ideas, I'm all ears like Spock.
A ban or removed comment will always have a direct link in the modlog, as far as I'm aware.
To find it, you should be able to go to the modlog, then click on the 'Filter by mod' selection, and search your own username. This should show all of the mod actions you have enacted. From there you should be able to see the comment you removed, which will itself be a clickable link, bringing you directly to the deleted comment on whatever post it was on. From there, you should be able to do the 3 dots thing and unban.