I got it working. Turns out for bedrock uploading the world is different than Java. And the https://support.aternos.org/hc…0027235751-Upload-a-world
doesn't mention that.
For bedrock the world is in
\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftWorlds
instead of in the .minecraft folder
Once the right world is uploaded I was able to join no problem