This is possible using any firmware. Be sure that your router’s settings are reflected in the control center.
Click Modify Hotspot Data & Settings – SPLASH PAGE SETTINGS and change the Internal Login/Logout URL Set: to custom.
Then enter the network address in the internal login/logout fields. Make sure that you keep the format for each field according to your firmware, for example for DDWRT:
http://#.#.#.#:3990/prelogin
http://#.#.#.#:3990/logoff
The same goes for your router, make sure that you keep the same format for the ip address field for example, DDwrt #.#.#.#/24.
The popup window will automatically point to the custom ip network for users to logout.
Note: when redirecting users after successful login there is no popup window, so you should set a low idle timeout value or advise them what link to visit for logging out.