Learn more about this change here.
For help regarding Aternos, we recommend you to visit our support center.
I couldn't add the mod Distant Horizons on aternos "This mod was only made for the Minecraft client and not for servers." - BS
PLEASE, add it. 
(also "Eating animation" "first person model" "not enough animations" "travelers titles" "optifine but faster" mods)
All the mod you mentioned are client-only. To use them you only need to install them on your client. Installing them on a server does nothing at best and causes your server to crash at worst.
