Just tried this but got "this device needs iOS 15 or later" when trying to install Microsoft rdp client, which isn't available for my old iPad. So I guess you can't actually use an old iPad for this. But maybe I can find another rdp client that will work.
Do you have the RD Client app installed on an iOS 15+ device? If you ‘own’ the app (and the developer allows it, I think) the App Store will let you install the latest supported version on an older iOS. I was able to install RD Client on my iOS 10 iPad that way a little while ago, but maybe something’s changed in the meantime.