I would like to get your feedback on the following design:
can RB750gr3 setup the ssid for WLAN router / AP? or should i set the ssid in the WiFi Router and treat RB750gr3 as WAN/internet gateway?
will RB750gr3 as DHCP server will be able to assigned IP to all the wifi AP client ?
As long as you use all wifi routers ONLY as accesspoint/switches, the Hex can provide DHCP services for all users.
As noted, its important to know if the wifi devices are vlan capable otherwise there is no way to use them for more than one subnet and one SSID, unless each is attached to a different port on the router. If that is not possible (and clearly you dont have enough ports) then suggest you replace the first wifi router with a smart switch and then run all four wifi devices as AccessPoint/swtiches off the smart switch. This will mean you can have 4 different SSIDSs WLANS for the most part.