Hello,
I have explained in the very first message that simple custom code won’t work for the input field inside the iframe. That answer was given by the third-party person, not by the plugin author. If you use the third-party plugin to implement the payment method or any other feature you need to contact plugin author to change things related to their plugin. Besides, if you read the answer that you posted http://prntscr.com/sjo6vt you know that plugin developers should change this, not theme author because we are not able to customize plugin files.
Please, solve this issue with the plugin author or find another plugin that does not have this issue.
Regards