@hp_jambucha 

Hi, if you want to replace the original antenna on the MR200, 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. Five 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. The connector on the MR200 is SMA female, you can search online, ensure the connector matches.
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.
5). Considering that the extension cable will cause signal attenuation, the longer the extension cable, the more signal attenuation, so it is not recommended using a very long extension cable.
Hopefully, that answers your question but let us know if you need more details.

@hp_jambucha 

We cannot provide you that info like which antenna is better, how much benefit or signal improvement you will get with the replaced antennas, etc., as our developer didn't test the performance with third-party antennas with the router.

As for the frequency of the MR200 v4, it is 790 MHZ-960 MHz and 1710 MHz-2690 MHz.

As for the distance between the two antennas, your understanding is correct, it is not suggested to put them too close to each other.