This should be a simple fix. In cable their set top boxes has a setup menu.
in this menu there is a screen resolution and it can be set to 780, 1080 etc. and higher set it to the highest.
there is also a ratio setting. Set it to the different ratios until your tv is showing up as filled in.
sorry I cannot help further since I don’t have Fios tv just internet. But that is the way I set up my tv’s years ago with cable. Should be the same procedure with Fios tv.