Amp is the last thing you should worry about. First be sure you are aiming correctly and that you are taking environmental and terrain issues into account. Most people don't realize this, but adding a tripod to your setup will greatly improve range because it will keep your aim steady.
If you want to improve the connection and you are inclined to modify the controller, be sure you are using high quality cable and you have a sound connection with the transmitter. Make sure you have a high quality antenna, you mount it well, and that you know how to aim it correctly.
You may choose to improve the antenna on the AC side too, if you can. But the stock DJI antennas on the AC are already very high quality. If you think you want to do that, it's up to you.
After that, you can consider an amp.
History has proven it is not wise to place an amp at both ends of the bridge. Obviously, with two signals, you will need two amps, and both should be on the pilot side. So if you use amps, put them on the pilot side and leave the AC alone. If the amp is between the transmitter and antenna, it will amplify that signal before it gets to the transmitting antenna, then it will send the amplified signal out over the air towards the receiver. If the amp is between the receiver and the antenna, it will amplify the incoming signal after it has been sensed by the antenna, improving the strength of the signal that the receiver would otherwise receive. If the amp is between a transceiver and antenna (such as on the true wifi Phantoms) it must be a two-way amp, and it will help both the outgoing and incoming connection.
IMO, Sunhans is the most reputable source for RC amps. Good luck!