Unfortunately, Aternos doesn't offer access via FTP or the ability to upload custom mods/plugins. There are many reasons for this decision but it's mainly for security reasons.
Plugins or mods uploaded to your server run on our hardware. They might contain suspicious software code which is then executed in our environment. This might be a bot or a type of virus. Although your server will be launched in a protected environment, it may still be possible for someone to try to bypass our limitations e.g. to keep the server online 24/7. That would not only be unfair for all other users but would also affect our queue and waiting time.
Some other files can be uploaded using the file manager on our website. This includes datapacks, Addons for Minecraft Bedrock Edition, and WorldEdit schematics.
We are aware that it would be possible to install systems like virus filters etc. However, these filters wouldn't detect code to bypass our limitations, because they are too game/platform specific. Also, a procedure like a manual check is practically impossible. The huge amount of users and the huge amount of mods/plugins etc. would require such a great effort which we cannot afford. Therefore, we prefer to focus on other topics that we think are more important.
That doesn't mean you can't suggest mods or plugins in general.
Have a look at this article: Suggest new plugins, mods or modpacks