TLauncher has problems connecting to Aternos servers because it's missing essential features.
Our recommended solution: Don't use TLauncher.
When using TLauncher you probably get a connection timeout error, it might look like this:
Connection timed out: no further information:
If you really want to use TLauncher, you have to connect using the address and the port of your Minecraft server. You can find the port of your server by clicking on the "Connect" button on the server page. In your launcher you have to enter the port after a ":" in the address field like this:
The port in this example is "12345".
If this still doesn't work, you might have to use the DynIP.
For anybody interested in more details: The problem with TLauncher is that it somehow modifies the default Minecraft client so that it no longer resolves SRV records, which is a DNS record that can include both the server address and the port. Our system is designed to use SRV records because we dynamically start servers on different hosts every time. You can read more about DNS and why we need it in our "Connect to your server" article.