How can I restore my old purchase?

If you bought the ad-free version of the app via an IAP (In App Purchase) or subscribed to Podcast Addict Premium, then installing the app from the Play Store account used for the payment will automatically restore them.
However if you bought the Donate app a couple of years ago (Before May 2020), you will need to open the app Ad-Free/Premium screen and use the link that allows you to Restore the old purchase

Previous purchase link
Press on the link to Restore your previous purchase

Another solution can be to open the Play Store app and then go into Library / Family Library using the navigation sidebar. There you should be able to find and install the Donate app.