Trying to Subscribe to Fing Premium on Google Play

I have a Samsung Note 20 and I have no problem purchasing apps or subscribing to services. In the Fing App it shows the box and I’m using carrier billing and then it says “Transaction is Declined”. It doesn’t give me a reason and I don’t know why. I have contacted TMobile, but they say it’s not a problem with the carrier billing but the application. Can I get some help?