I just signed up for Fios and immediately noted drop-off in mobile devices. When I move farther away from router, the router would downshift to 2.4ghz from 5ghz, but wouldn't shift back as I moved closer to the router again.
The router can't manage the changes very well from low speed 2.4ghz to 5ghz and back again (although my old Netgear Nighthawk had no problem).
So I turned off SON in advanced settings and, so far, have not downshifted. While I keep the 5ghz connection moving around the house, I still do lose speeds. The lower speeds are still better than 2.4ghz.