Hello,
On the order page, the payment options displayed are those that are installed, active, and registered to the paymentOptions hook by the module.
Please refer to the following screenshots for reference:
If you want to display the Redsys module, you need to check and update the relevant function, as this function prepares the list of available payment options:
https://github.com/PrestaShop/PrestaShop/blob/8.2.0/classes/PaymentModule.php#L887
Note: This function is used throughout the PrestaShop system, so any changes should include the necessary conditions to ensure compatibility across all pages.
Best regards.