I have been facing the same issue from last 3 days or so. It is working for me with
DNS IPs 4.2.2.2 and 103.250.184.85. When I used the combination of netstat and nslookup for working and non-working scenario (after initiating a download), it seems that the IP I am being resolved to are different in both scenarios. It is not working even with otherwise excellent
google or openDNS servers. I could only find these two IPs which are making it work. It looks as if
Apple has placed any kind of QoS policing to throttle down the rates for connections initiated by Airtel public IP pool and destined to their that specific server VIP. Try using one of these two IPs as your DNS server and let me know if it helps. You can use any IP from 4.2.2.1 - 6.
Thanks,
Piyush