I will try to be more precise.
When a visitor comes to the reception, an employee gives him a visitor wifi by connecting to the "Instant guest users" interface.
The guest connects to the "captive portal" and inserts his username and password that has been provided.
The username contains the company name and a random password.
The problem is that when the employee creates the visitor account via "Instant guest users", the credentials created by the employee and the credentials provided by the employee to the visitor may be different due to an employee error. The visitor wants to log in and the username and password are not correct.
When you say that we can use clearpass, I suppose you are talking about the ClearPass Guest module which will have an additional cost?
Regarding an external web page, can you tell me more?
Thank you in advance for your answer.