Hello,
I believe that only versions 8 or 10 of the Microsoft Remote Desktop app are compatible with your iOS version. Unfortunately, I have searched extensively, including third-party archive websites, but I was unable to find such an old app package.
Check if you can find an older beta version through TestFlight. As older beta versions for macOS are often available for download on the Microsoft website Microsoft AppCenter.
I would also suggest checking other remote desktop clients that may be compatible with your device.
I apologize for the inconvenience.
Best wishes,
Karlie Weng
If the Answer is helpful, please click Accept Answer and upvote it.