I've been fighting this a long while. Every time I load up an old AC game I've had issues trying to get it to connect to the Ubisoft Servers in order to play with the digital content.
One thing that, so far, has seemed to work consistently is that is use Ubisoft Connect, come to these forums, which have invariably logged me out, and I log back in.
I then open the game, and it seems to, so far at least, automatically connect me to the Ubisoft Servers and give me access to the digital content.
Maybe I've just been lucky, since in the past I had even resorted to opening and closing the game until it connected, but fingers crossed this may be the easiest solution.
Come here, log into your Ubisoft account (not just Ubisoft Connect), and then open your game.