In game messages are very hit and miss especially when it comes to the more active players. The game sends many automated messages to the player based on certain things like contract setups even to yourself or enterprise building. I get dozens of messages a day in my inbox and they're literally just system messages from normal country management. I easily could have received numerous messages from players using the in-game messages and not seen them because most days I don't even look at the messages because of the quantity of system messages. Discord I feel is the best way to message communicate with the other players in game.
If any new players see this, seek out Doktor Doom on Fearless Blue if you can't reach anyone else. I always check my messages, and will teach you how to get started & build defenses. Always willing to help new players!