Learn more about this change here.
For help regarding Aternos, we recommend you to visit our support center.
Make sure you are using the mod also on your client.
And also check the version if its the same. What message you get on your screen?
You can download the forge on your client to make it work.
Have your friends used Forge version for that?
In your first message you write about CustomNPC's and GalacticCraft. Which mod do you play now?
Your client must be on 1.12.2 on CustomNPC's 29oct17 must be installed.
What error message do you get when you try to join your server? Please post a screenshot.
Also your server log would be helpful. Click the share button on https://aternos.org/log after trying to join your server and paste the link here.
