I got Xcode 12.5 (I guess it’s the last version).
I’m in fact using a Mac through TeamViewer (a friend of mine borrows me his mac). And my project is currently using blueprint only BUT we are planning to release in the end an app using C++, that’s why I wanted to “test” the deployment directly with a Mac, so I’m aware of all of that.
Anyway, I also understood that even if you have Blueprint only or c++, you need a mac in the end to deploy it to Testflight and Appstore. Soooo that’s why I used a Mac.
I’ll take a look at what you said about renaming my ipa file, as this is the last thing I need to try.
What I’m most worried about, is that in your case, you seems to get this workaround working whereas I can’t on my side. That’s…strange 
EDIT: I even tried to upgrade my project to 4.26 and specify the “Minimum iOS version” to 14.0 and it’s not working, as it’s still saying that my app is with iOS 13.2 SDK… That would be awesome if Epic could update this on their side…