@ChSy 

What is the model number of the AC1200? It starts with Archer. 

The antenna is used to convert radio frequency (RF) fields into alternating current or vice versa. It includes both receiving and transmission antennas. 

While it has no direct connection with speed, instead, it is still affected by the bandwidth of the sim card and the actual network environment. 

You can test the speed you can get from the sim card with putting it into your cellphone directly.

And you can change some wireless settings to enhance its performance, like wireless channel and channel width.

May it help and have a good day. 

@ChSy 

Hello, Chris, thanks for your reply.

I haven't heard the Poynting antennas you mentioned, while if you want to change the external antenna, we need change all of them. And the Archer MR600 has 2 × Detachable 4G LTE Antennas, so it is possible to replace them.

From your description, the speed you can get is more related to the environment or the coverage area of the 4G base station.

Per you said, the higher you stand, the faster speed you can get, that means the signal has been cut a lot by the interferences. Including distance, obstacles. 

In other word, you can change the external LTE antenna, while we cannot guarantee that the speed will reach more than 20Mbps. 

May it help and have a good day.