@nassos69 

Hi, we are not sure if you can buy the original antennas for the MR6400 in the local market, you can check with the local sales team or check with the local stores.

If you cannot get the original antenna, then you may want to buy something compatible, please keep this in mind when purchasing. The two external antennas are 4G LTE antennas, they are different from the ordinary wireless antennas. These antennas are mainly used to receive 4G signals from your mobile ISP by auxiliary SIM card. Four tips:
1). You need to change the two at the same time, and the two replacement antennas should have the same specifications. If you only change one of them, it will not achieve the desired effect, but the effect may be worse.
2). For the replaced antenna, the interface type needs to be matched. The interface type of the two external antennas is SMA plug, that is SMA male, not ordinary RP-SMA.
3). The frequency range supported by the replacement antennas should be the same as the original antennas.
4). The distance between the two replacement antennas shall not be less than the distance between the original antennas.

Hopefully, that answers your question but let us know if you need more details.