A learning remote would probably be your best option. You should be able to use just about any universal remote as well, but you'll need to enter the brand codes manually until you get the right one.
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According to the Harmony remote database they do support the Stello Ai500. You'll have to enter the model number as I've typed it here, with a lower case i.
You can connect any of the Harmony remotes to a computer, go to this url, https://support.myharmony.com/en-us/compatibility and download the remote codes.
zeal -- Yes, you'd need a computer to download the codes from the Harmony database.
You attach the remote to a USB port on the computer, access the Harmony database, go to the page for your piece of gear and download the codes for access from any of the buttons you want on the remote. The process is relatively painless and works very well.
Your alternative is, as mgreen27 suggested, to manually enter the code for your Ai500. The problem with that is that you're not going to find the code in any standard list of device codes because those typically list codes only for common consumer-level devices.
If April Music can provide the proper code that should work fine but if not, manually entering codes from thousands of possibilities until you find the right one is a tedious job with the prospect of never finding the right one.