Fios won't let me sign into my Apple TV on a Channel I already get through my set box.
You could remove the channel from Apple TV and then reinstall it.
Apple support will not help much since they refer you to the app maker. Who is responsible. Now when you said Fios that would alert me that your credentials may not be exact (logon or password or both) try them again or call 1-800-VERIZON and speak to tech support. It may be a simple fix.