The app says I need to subscribe
What it means and what to do—trial ended, restore subscription, or subscribe.
If SmishAlert is asking you to subscribe, it usually means you don't have an active subscription yet, or you're on a new device and the app doesn't know you already subscribed. Both are easy to fix—subscribe in the app or tap Restore purchases.
My trial just ended—I want to keep using SmishAlert
Open the main SmishAlert app (the green icon on your home screen) and tap the option to subscribe. You'll go through Apple's payment—$49.99/year. After that, everything works again. No need to change any Settings.
I already paid—I have a new phone or I reinstalled the app
You don't have to pay again. Open the SmishAlert app and look for "Restore purchases" or "Restore subscription" (often on the subscription screen or when it asks you to subscribe). Tap it and sign in with the same Apple ID you used when you subscribed. Your subscription will come back. See Restore your subscription on a new device for full steps.
More detail
For a longer explanation of the "Subscribe to continue" message and what to do, see Why do I see 'Subscribe to continue'? in Subscriptions & Billing.