Redmi support only few frequency bandwidth and the MSP has various bands at different locations.
Vodafone and Airtel have started puahing 4g speeds through 3g bands. They have started 4g services using band1 and that band is rarely supported by any other mobile phones. That's why this issue is rising. But don't worry about it as chipset supports this band and it can unlocked by fastboot. So wait for the solution.
Yep they started using their 2100Mhz for 4G services, because they have free channels in that band....They just have to spend money for setting 4G supported equipments on their already established network towers...No need to buy new spectrum in that circle....Saving money..
Sorry for late information,
But Vodafone have started their 4G services on band 1 which is creating these problems as too many other brands users are also complaining for the same. So I will suggest you to hope for something better from Mi India side.