Can I increase signal strength on my Samsung Galaxy s4 active?

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http://android.stackexchange.com – I recently purchased a used Samsung Galaxy S4 that used to be on AT&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? (HowTos)