For now (9/2/2017) problem occurs on Open Beta showing NAT with red letters top left quarter of the screen, while in game. This means that your router's NAT type is strict or firewall and antivirus block the game (or both).
Solution: Navigate to Start -> Control Panel -> Windows Firewall -> Allow an app or feature through Windows Firewall
Check if uplay or uplay launcher is included in the list. If it is NOT included then click on Change Settings -> Allow another app... -> Browse -> X:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\uplay.exe and click Open
NOTE: X is your storage drive letter. If it's not in "Program Files (x86)" try "Program Files" or find the location you installed uplay.
Then click Add.
Router Changes:
You need to navigate to your router through your browser or dedicated app and forward the following. If router requires start and end port or internal and external port put the same number everywhere.
E.g. first rule would be
