I am using RouterBoard RB951Ui-2HnD. I want to run capture portal on WLAN and keep internet connectivity on LAN. WLAN wont have internet connectivity. I am thinking about disabling NAT on WLAN and keeping connectivity on LAN but I don't know how to do this.I am using winbox .Please help I am new to this.Thanks in advance.

You cannot disable NAT on your WLAN interface except you're doing dynamic routing. Check if your WLAN mode is on AP_BRIDGE. Then Add Hotspot to your WLAN Interface.