You need to log in with your own account with whatever cable service you subscribe to. Alternatively, it's available on Playstation Vue and YouTube TV.
Roku is indeed remembering what your credentials are for the Fox family of apps. You an only use one for the whole suite of apps.
You'll need to log out of all of them. -- FoxNow, Fox Sports, FXNow, and any others.
Once you do that, you might want to remove the apps.
Next, and this is critical, reboot the Roku.
After you've removed all the Fox apps, then rebooted the Roku, clear your browser from all cookies and local storage for any *fox* or *fx* Website.
Then, clear your browser infor for any *directv* or *att* Website.
Now, you should be able to log in to the desired DirecTV Now account in your browser.
Finally, add one -- only one -- Fox app back to the Roku. Authenticate against DirecTV Now.
Does it work? If so, add the next Fox family app.
If not, report back.
Anyways, I had removed the FX Now and Nat Geo TV apps months ago from my family's Rokus since I had heard they've been merged into Fox Now, is this incorrect? ...
... To make matters even more confusing, will all this change soon when Disney acquires the Fox cable networks? Will the FX and Nat Geo content be "un-merged" so that the Fox Now app will only have the Fox broadcast content, plus maybe Fox Sports?