Seems broken
Just had need to set some new parameters, seems like iOS 15 breaks the entry field of the app. You can still manually set things via SMS, however.
Not enough ratings yet to assess SMS Alert.
SMS Alert appears to be a legitimate app. AppsHunter Legitimacy Score for SMS Alert is 80/100.
Based on 80 of 100 points across 7 verification signals from App Store and developer data.