Installed FiOS mobile app on my wife's iPad and my son's Android tablets. Both have their own Vzn login (aka sub-account) and both sub-accounts are checked as "✔ This account can access FiOS TV Online" in each's profile settings (via Vzn's residential acct web mgmt interface) (In fact, my wife is noted as an administrator.)
Logging in from either tablet produces the following message for both:
And when I click "Manage Sub Account Access link:
the webpage responds "We are unable to process your request at this time."