Verify the permissions requested by the app (a ringtone app should only need storage access, not your contacts or location). Step-by-Step Guide: How to Cut and Set Your Own Ringtone
Once you have downloaded the MP3 file to your device, follow these steps: On Android app and go to Sound & Vibration and tap the (plus) icon or Add from phone wo mera nabi mera nabi mera nabi hai ringtone download mp3