Need help uploading to app store with older Mac

I have been trying to upload an app, and I have a Mac but it's from 2011 and the most recent Xcode I can install is XCode 9.2. I cannot install the new Transporter app, either.

My solution for building the app was Unity Cloud Build. That way I have a .ipa file built with a recent enough XCode. However, I have not been able to upload it to the app store with this computer. I tried the Application Loader in XCode 9, which appears to work, but the builds don't show up in App Store Connect.

Can I upload from my old iMac, or do I need to borrow a newer computer to do it? Is there some step that I'm missing?