RB750 As Captive Portal with existing access point

So I just received 2 RB750’s.

Based on previous questions I have been told that what I want to do is very possible. Now comes the difficult part. How to configure.

I currently have a firewall/DHCP server in place. All I want to use the RB750 for is Captive Portal with external radius authentication for Wifi.

I want to be able to take the RB750 and plug it into the current lan subnet. I want to connect the access point to one of the RB750’s ethernet ports and have all wifi connections made via the access point to get an IP on the same ubnet but be redirected to the captive portal page.

How is this accomplished? I am brand new to this since I was previously a die hard pfSense user…

Thanks,

Mark