Can I increase signal strength on my Samsung Galaxy s4 active? I recently purchased a used Samsung Galaxy S4 that used to be on AT&amp;T, but it has been unlocked, or something so that I can now use it on Koodo (canadian carrier).

The signal is very weak.  It's so weak that talking on the phone has been a hassle because people can't really hear me and I can't hear them.

I found out how to switch from LTE to WCDMA, which makes the signal a little better, but then i miss out on the benefits of LTE.

Is it possible that the phone is not running on the optimum frequency band for usage on the Koodo network?

Any suggestions or help?
When connected to LTE, the phone is also separately connected to the GSM network for voice. Voice does not go over LTE so switching your data to WCDMA (HSPA) won't make any difference in the quality of voice calls, unless you are using VOIP calling. 

There isn't much you can do to boost the signal other than not using a case or if you do, stick to a thin TPU case that is just colored (ie no carbon fiber or chrome or wood or metal). If it is at your home, you may be able to see if your carrier offers a network extender. The network extender plugs into your home Internet and emulates a small cell tower in your house.