My understanding is that for roaming two access points only need (a) be connected to the same wired L2 network (b) have the same SSID (c) have the same security setting.
I am using a Linksys WRT1200AC and a TP-Link EAP610 but roaming does not work: Devices stay connected to the EAP610, even if I am standing right next to the Linksys. iPhones wouldn't even connect: "Unable to connect to this network". Even if I unplug the EAP, nothing changes. Only if I remove the wifi network profile completely from the client devices, re-connect and re-enter the password, devices can connect to the Linksys again.
These are the settings for Linksys WRT1200AC with OpenWRT:
And the settings for the TP-Link EAP610:
