User Task Due Date not working

or is there any alternative to this?

Using a process variable, you can set a dynamic value.

For example, a manager can set the due date value to user-defined variable through the form. And this value will be used for the next task.

A task listener can be used to calculate a due date as well: https://docs.camunda.org/manual/7.8/user-guide/process-engine/delegation-code/#task-listener

It can be implemented in a script.

Cheers, Ingo