Be careful with that older phone, many of the older ones have lousy radios, and don't cover all the frequencies used today. So, even going to a sleek 4G still might give you a problem.
Im on Verizon unlimited so I don't get free phones so I used my Galaxy Nexus on the Sleek 4G and didn't have much increase in speed if any. I put my wife's Samsung S5 in it and it flies with the Sleek, more bars and much higher speed.
As others have said buy a new antenna that covers all the 4G frequencies.
On edit, what's the version of your wife's phone?
Ron W.