I’ve bought Mini Metro on the App Store previously, but I can’t redownload it without purchasing it again.

Could you please try the following on your device:

1. Open the App Store.
2. Tap on the profile icon on the top-right.
3. Tap ‘Purchased’.
4. Search for ‘Mini Metro’.

Does it show up on the list, and are you able to download it? If not, there are number of others things to try out, which you can find detailed at https://appletoolbox.com/2012/08/purchased-ios-apps-not-appearing-on-app-store-purchased-tab-ipad-iphone-or-ipod-touch/.