Sorry for reposting this, but the old thread was closed and so I was unable to edit it.
Please add the Player Tracking mod to Aternos. Here's the curseforge page: https://www.curseforge.com/minecraft/mc-mods/player-tracking
The mod adds a way to track other players using a "compass" you will have to build out of expensive materials. It is meant for faction servers or PvP based survival servers, so you can track down other players and raid their base.
Thanks.