Online Hacking not working

The answer showed up just after I posted this question.

As I reached the location for the next mission ("Backseat Driver") and attempted to start the mission, I was informed that another fixer was attempting to hack me.

The game then required me to navigate to an area indicated on the map by a purple circle. When I reached that location (for me, this was the parking garage where I'd just completed a side-mission) I had to scan people until I found the rival fixer (it appears that this is always a bot called 2XTheTap) and chase him down and shoot him.

After this, I was able to access both the Online Hacking and Online Tailing modes.

Some research shows this is the standard way to unlock this feature.

I disabled every network adapter I had installed except for the one required for internet access.

I had VMWare Workstation installed, the adapters of which seemed to be the culprit.

I don't recommend forwarding ports unless you're aware of the security implementations of doing such (especially :80 and :443).