UPI's are not working in Iphone 6 IOS 12.5
Please fix this issue soon. It's been a while now. I am still not able to use my iPhone for UPI payments.
It doesn't make sense to buy a new mobile phone for UPI payment.
iPhone 6
If you want to keep your transactions safe, it does make sense to buy new iPhone models.
See the pic below which clearly states you need iPhone 7 for UPI.
Every app has its own boundaries. Please contact the app developer --> How to contact an app developer - Apple Support