Biomes O' Plenty is a mod that modifies the world generation and adds several new biomes to it.
First, you have to switch your server to the forge server software (https://aternos.org/software/forge/) and install the mod on your server: https://aternos.org/addons/a/curseforge/biomes-o-plenty
If you're using 1.18+ you additionally need to install the mod TerraBlender.
On versions before 1.18, you then need to generate a Biomes O' Plenty world for your server.
MINECRAFT 1.12.2 and older: Type "BIOMESOP" into the worldtype field.
MINECRAFT 1.13.2-1.17: Type "biomesoplenty" into the worldtype field.
MINECRAFT 1.18+: It's no longer required to use a custom world type
Complete the process by clicking the generate button. Please note that your current world will be deleted.
As the last step, you only need to start your server.
Please note that you need to install forge and the Biomes O' Plenty mod on the client too.