Just ran into a strange problem. I do a standard Hotspot installation and follow the instructions for an external login page.
It all works fine the first time I connect. When I disconnect from the Wifi hotspot and connect again the Hotspot does not redirect me to the external page and says it cannot be found.
When I reboot the router it works the first time I connect and then when I disconnect and connect again it does not redirect me and no access to the external login page. Wonder if it's a Firewall problem?
I am using a RB 951UI - 2HND - RouterOS v. 6.33 (stable)

Did you add the ip address of the external login page to the walled garden of the hotpot?

Can you try to connect with other devices after disconnecting the first one?