Thank you for posting your concern in the Community; undoubtedly having a reliable network connection while travelling abroad is quite important.
I appreciate you resetting network settings and trying other troubleshooting so far. Have you checked the carrier to which the phone is defaulting to? If it’s roaming on Airtel, then please manually switch to another operator within the device’s network settings.
Please provide us with your device details if you need assistance in manually switching to a different operator.