Using the input: 'range' may have the side effect to be difficult to use on small screen / touch devices. On such devices is more difficult to provide a fine grained control on the input. The 'range' input in Swal2 is designed to have only an output and no input.
Alternatively, a range input with a number input can be achived manually using the html property to add a number input. In the didOpen() function, the two inputs can be linked together using addEventListener.