Airtel 4G in Delhi / NCR is on both 2300 MHz (Band 40 - TDD) and 1800 MHz (Band 3 - FDD). Here is an excerpt from a newspaper on the subject:
?Today, as we launch Airtel 4G trials exclusively for our existing customers in Delhi NCR ? our customers in the region will be uniquely positioned to experience India?s first integrated FD and TD 4G network,? Bharti Airtel (India) Hub CEO (Delhi NCR and Rajasthan) Sarang Kanade said. According to sources, Ericsson is providing the network for FD (1800 Mhz) and Huawei for TD (2300 MHz) network.
It is a ROM update issue with India specific Lumia 1520. It will need a ROM update to unlock 1800MHz FDD LTE capability on Lumia 1520. It works on my Lumia 1520 flashed with Singapore ROM and I am able to connect to 4G. 4G shows up in the option for available maximum data speed. Also, as I have installed latest Windows Mobile 10 Insider preview (10149) on my phone, I get the following options in the max data speed:
2G, 3G Only, 3G, 4G Only, 4G.
Microsoft needs to update the ROM ASAP now that operators are beginning to roll out 1800MHz FDD LTE.