Their is some software programming required to be done with unlocking of ZTE AC2766 to work with BSNL EVDO . Only seller braham3997 is selling unlocked ZTE 2766 which works with BSNL EVDO when i had last check ebay .@kuduku According to @hA1d3R , his unlocked ZTE AC2766 does not work with his TP-Link MR3220. He has posted about this on the TP-Link forum too.
http://forum.tp-link.com/showthread.php?73626-ZTE-AC2766-MTS-Unlocked-Not-Working-Need-Bin-File
Did you use the unlocked ZTE AC2766 that you bought off eBay from braham3997 with your MR3020? Is your MR3020 flashed with DD-WRT?
I had purchased from seller after confirming that it works with EVDO then posted the link . I bought it for 700 bucks and see the price now . Supply and demand
I have another Buffalo dual band router with Shibby tomato and in that ZTE AC 2766 works fine . My MR 3020 is with original TP link firmware .
ZTE AC 2766 is supported with TP link as shown on TP link compatibility site . The moment RUIM is unlocked by users then the issues of not compatible have been reported .