Fortigate 50B appliance with routing -> Captive Portal -> Internet

I want to replace the Fortigate with my RB450 (6.12) because of the following issue:

When users on the LAN side of the router (192.168.30.1-254) login the captive portal captures the WAN ip of the router (192.168.10.9 and MAC address). So after the first person logs in everyone else get a free pass.

I am guessing that it has something to do with NAT. the current router only does NAT or Transparent. If I set it to transparent I believe everyone has to be on the same subnet (can't have that). So do I set the RB450 to route without NAT? Would this solve my issue? If so How do I do this?

Maybe, yes, depending on your configuration. You probably dont need to be NAT'ing on the LAN side of the Hotspot. The Hotspot will do that anyway.