Generally I try to not slam people who post "it works good for me" when they have no clue about what they are posting. In this case he has confused a number of people into thinking that all airtel dongles work on tl-mr3430 routers. I suspect it is because he does not have a clue there are many many models or airtel dongles and the part number makes a huge difference. He likely happens to have one on the supported list so of course it works.
This is the list for the mr3420. I did not find the device you want to use in the list. There is a tiny chance this might work because it appears the same as other huawei devices but I would not take the chance.
http://www.tp-link.com/en/support/3g-comp-list/?model=tl-mr3420
If you have not purchased the router I would look for a different model. I really wish they would standardize the modem interface so it would be easier but unfortunately what you are going to have to do is dig though all these huge lists of supported dongles until you find one that matches your router and dongle. Likely the simplest way is to call your ISP and ask them for a recommendation.