Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
Apple's make the encryption password ***** to break. You probably set this password ***** you backup the iPhone first time. So I want you to try the following passwords.
4 or 6 digit passcode of the old iPhone
Last 4 or 6 digits of the phone number of old iPhone.
Apple ID password ***** old Iphone
Does it work ?
Do you still have the 5S with you ?
Are you using an Apple Mac or Windows PC ?
If you are using a Mac computer, then we can try to get the encryption password.