In cases where your server isn't appearing via the server browser it will be worth using the "Add Server" option within game.

This will enable you to find the server and add it directly into your favourites section.
In this case you will want to use the IP and Admin Port.

Eco doesn't use its normal game port for the connections at this time.

In this example:
Admin Port: 10015

Once you have these you can launch the game, select "JOIN

Here you want to select "Add Server

Then enter the "IP:PORT"

Once this has been done you will find the server under the "Favorites" listing as seen below