You have to forward the required Port(s) in order to play. If you have already done that, your local IP address has probably changed and your forwarded port points to the wrong local ip-address. Please use a static ip-address or configure a fixed ip-address in your router settings. The game itself but also Voobly client must also be allowed through any firewall and anti-virus, also check, that you only have one firewall active e. g. Windows Firewall.
Unable to join usually indicates a connection problem, e. g. a firewall either Windows Firewall or some Internet Security software. Make sure Age of Empires II is allowed to access the internet.
Please follow the steps as explained in this article: KB1011 Ports that you must forward to play or to host a multiplayer game on Voobly