@SujitKrSahoo 

Yes, for best results both antennas MUST be connected.

1 external antenna only & 1 internal antenna will work but usually in Non - MIMO mode, (if the signal to internal antenna is low) for MIMO mode to work you require 2 internal antennas (if the signal is high) & for a best results, 2 external antennas are required, it all depends on your signal strength received, i.e how far you are from the cell tower/s.

A higher signal strength on both streams (4G+ MIMO) means higher speeds (Mbps).

Read this for more information ....  https://community.tp-link.com/en/home/forum/topic/272342

John vk3hjq

I thought you said it was only working on 1 antenna, & that was your problem in the 1st place.

Now for this Question .....For this to happen (& it can) the signal strength must be very high, i.e you are very close to the cell tower.

It is not a good idea to run it with only 1 antenna connected as it is designed to run with 2.

John vk3hjq