You have a choice, right?
No, you literally don't. Every iPhone app is expected to work on an iPad. It doesn't have to be written as an iPad app, i.e. it doesn't have to take up the full screen and have all the same navigation stacks, tabs and layout as an iPad app does, but the app must function on an iPad.
App Store Connect shows it is an iPhone app, yes, but that just means it is listed in the App Store for iPhones. If someone downloads it on their iPhone it will also become available on their iPad, too, so it must work on their iPad as well.
As I said, run it on an iPad and see if the app works properly. If it doesn't, then you have to fix it. The App Review Team are literally telling you that they are rejecting it because it doesn't function properly on an iPad.
Your app must work on an iPad, so make it work on an iPad. Just run it on an iPad and see what needs fixing. It might be a really simple thing to fix.