Are you planning to use another router with it (putting Airtel provided one into bridge mode?)
If yes, then get the default Sercomm (the black one that looks like a DTH box). It is pretty easy to configure and range is good. Only downside is, if you plan to use it as the main router, then you can't configure more than 8 port forwarding rules (as it was on the firmware when I tested. Not sure if it has been updated)
Nokia has good range but it's admin panel takes forever to load and to apply changes.
ZTE and Huawei are good too (I think Huawei has only 2 LAN ports)