Actually, Beth, Hans appears to be right. It's no longer available for purchase in the Store.
You can see it because you've purchased it. I can't see it, because I don't have the Pro key - I get Pro access through having the monthly subscription
You see this a lot in the Play Store - a lot of devs refer to the method you've used, Beth, as "legacy". If you buy it as an 'app', it's subject to Google's Family sharing rules, which means that anyone you've defined as part of your family gets it for free.
The "newer" approach of In-App Purchases doesn't apply for other Family members - they'd have to buy it too. For that reason (at least, I'm pretty sure that's why), devs seem to prefer the In-App Purchase approach.
I assume in this case, the dev has removed the standalone 'unlocker' app for purchase in the store, meaning you have to use the IAP. It has to be visible for people like you who've purchased it already to allow you to carry on using it.
The other difference though is that standalone 'unlocker' apps have to be installed in addition to the app they unlock. That's not the case with IAPs. They just automatically apply. So, as long as you're installing the app from the Play Store on your new phone using the same Google account, you'll automatically get the IAP. (Of course, if you're not using the same Google account, you wouldn't have seen the unlocker either).
Unfortunately, Google don't make it easy to see your In-App purchases. You can see your Order History, which is similar.