Before initiating the restore purchase, let me explain that iman Pro consists of 2 payment models. They are:
a) iman Pro - One-time purchase from AppStore
b) iman Pro Premium - A subscription from in-app for additional weather information (monthly or yearly subscriptions) on top of (a) Read: https://iman7.adiman.net/index.php/iman-pro-premium/
Restore purchase should be used when if iman Pro Premium subscription (item b) was not detected by the app due to app re-installation or anything happened that prevents the access of premium contents.
It has nothing to do with item (a)
And if you subscribed to (b) Please check if your subscription is successful. Any successful subscription will receive a receipt from Apple. You can also check it in your purchase history. Read more on how to check subscriptions or cancel subscriptions on AppStore: https://support.apple.com/en-my/HT202039
Again, restore purchase does not work with (a) or when your subscription is already cancelled/expired or refunded.