setting up a new phone and need to retrive data from old backup

I have a question that I can't seem to find an official solution, will try to keep this post short and hopefully get some insight from here,

I was using an app called "balance" since 2007 on my iphone 3G, 3GS, 5, 6...

I have some balance data which I would like to keep a copy, if possible.

Just got a new iPhone X, did a back up on iphone 6 and restore on iPhone X. While everything is great, on the iPhoneX it shows the app is no longer available on app store - that is I can see the app icon but when I tap it is says "Balance" Needs to be Updated. This app needs to be updated by the developer to work on this versions of iOS. (and unfortunately I do not have the app-.ipa backed up on the computer)

Did a little research and realized the app is actually no longer available since 2012.

Guess what happen was since iPhone 3G I have always have the app on the phones when I upgrade so it always work on my "next phone" and worked as well on my iphone 6 ios 10.

However with iphoneX ios the app no longer supported/available on app store.

Worst is a also factory reset my iphone6 so restore my iphone 6 - I need to re-download all the app, so it comes to trouble as the balance all disappear on app store.

I assume the app data itself is still available on the iTunes backup file - only missing the app to actually retrieve the data.

Question - how that I have an iphone 6, with app data restored on the phone, anyway at all I can get a copy of that app and install back on that iphone 6 so I can access to the data?

Jailbreaking maybe one option, but I'm not sure if I can find the .ipa, also not sure when it gets jailbroken - should that wipe the app data I have.

Here is the app:

http://www.cwakamo.com/balance/

Many thanks