@Santosh7.-_Safi 

I would have thought that you would get a signal on both 4G Antenna ports, the SMA Antenna sockets are the Main (ANT 1) & the other is the AUX (ANT 2), maybe it faulty, I dont know, maybe its not MIMO and that's why & you need 2 antennas connected, & that's what happens when you are only connected to the Main Antenna socket (ANT 1), i am still learning about these modems myself, if anyone else has any insight that would be good.

What you could do it open it up & see (this Voids the Warranty) if maybe 1 of the wires has come off the 4G LTE Modem (Grey wires), here is what is inside an MR6400 for reference, hope this helps.

Edited: Confirmed by TP-Link, it will not work with ONLY 1 Antenna connected to the AUX Antenna in. (NO Signal)

https://community.tp-link.com/en/home/forum/topic/272342

https://community.tp-link.com/en/home/forum/topic/265604

John vk3hjq

@TP-Link 

Thank you for your reply.

John vk3hjq

@TP-Link 

What abt solution of my problem ?

Hi all,

Looks like you can use 1 external Antenna (Not MIMO though), it must be connected to the Main (ANT 1) Antenna in socket, & you still must have the standard Antenna supplied connected to the other AUX Antenna in socket as well.

P.S. looks like having only 1 Antenna connected to the AUX Antenna in, just does not work, can someone please confirm this is the case or not.

EDITED: Confirmed by TP-Link, it will not work with ONLY 1 Antenna connected to the AUX Antenna in socket. (No Signal)

John vk3hjq

@TP-Link 

Thanks for confirming this.

John vk3hjq

Very helpful info. I've been struggling to understand why there is 42kohms at the main aerial port and an open circuit at the auxiliary. I swapped small aerials around and found that as mentioned, only the main aerial port activates the modem and all lights show. I guess the question would be, what does the aux aerial do, why does it not show a 42kohms circuit, how does it handle carrier aggregation? I can't achieve more than 10Mbps in a 5G area, even with a quality directional external antenna. my mobile gets 50Mbps.

@Scubajoey 

Scubajoey wrote

Very helpful info. I've been struggling to understand why there is 42kohms at the main aerial port and an open circuit at the auxiliary. I swapped small aerials around and found that as mentioned, only the main aerial port activates the modem and all lights show. I guess the question would be, what does the aux aerial do, why does it not show a 42kohms circuit, how does it handle carrier aggregation? I can't achieve more than 10Mbps in a 5G area, even with a quality directional external antenna. my mobile gets 50Mbps.

https://community.tp-link.com/en/home/forum/topic/272342

John vk3hjq