The server overloading message can be ignored as long as the TPS stays at/around 20. That has nothing to do with networking.
“Connection timed out: No further information” just means the player took too long to connect.
On both the client and the server, you can add the following Java arg to lengthen the initial join period.
-Dfml.readTimeout=180
In AMP you just add it under Java and Memory, and for the client side, you will need to google “{insert client name} how to add startup Java arguments” and put it there.