Welcome to the world of mods. i have had some success on my own, by having another user keep the chunk a player is in loaded, when he joins. Try that. If not Share Your logs from when it happens here please.
1.) Go to your log on Aternos: https://aternos.org/log/
2.) Click on the big blue "Share" button on the top right
3.) Copy the link with (Ctrl + C or click on the "Copy" button)
4.) Send the link here.