Seamless Roaming between access points

Hello.

I have two AC1900 EX6400 set as Access points between second floor and a basement (Wired connected via Moca 2.0)

I don't think I get Zero roaming as the connection to the access point stays with the initial Access point.

I also have the G1100 Fios Quantum as the Cable Modem + wireless Router, but I could turn that off.

Is my current ex6400 supports zero handoff (Roaming), if yes then could it be that I need to play with the signal strength?

If no, then which devices I need to use as access points to achieve the zero handoff.

The devices needs the zero handoff are iPhones