Please help me find the cheapest compatible phone for my needs

Hi, I was given an early Christmas gift of a Halo Neuroscience Sport. It requires an app to be installed on a smart phone, only the phone I have is a Samsung Galaxy S4 ver. 4.2 and can't be upgraded.

My phone is a cheap refurbished model on the Straight Talk flat $45 per month plan. I can't afford an expensive phone/plan and have never needed one as I only use it to make calls and navigate. But if I don't get a compatible phone for the Halo device, I will have to return the gift. I would really like to keep it to help with learning the piano.

Unfortunately, I am dumb when it comes to smart phones. I see cheap unlocked straight talk phones on eBay, but I'm wary of purchasing something this way...most of them are from overseas and would be expensive to return.

Please let me know what I can do to solve this. I only have a little time left to return the device, as you can use it on trial for 30 day. The Experts really know their stuff, that is why I subscribe! Thanks in advance.

Here is the link to the models and OS versions compatible with the device:
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Shaun VermaakTechnical SpecialistCommented:
Any idea why can't you upgrade your Samsung Galaxy S4 to Marshmallow?
sheana11Author Commented:
Hi, yes, it is CDMA ....I asked this in a previous question, and the Expert checked it cannot be upgraded.
Shaun VermaakTechnical SpecialistCommented:
Even if you root it?
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sheana11Author Commented:
Shaun Vermaak, how do I root it?
Shaun VermaakTechnical SpecialistCommented:
Use at own risk. After rooting you might be able to upgrade

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sheana11Author Commented:
Hi Shaun, I'm closing this question with the rooting instructions, as that will take  a little time. If it works, great, I'll come back and let you know. You have provided an educational service for me, and that deserves points!  Thanks so much!
sheana11Author Commented:
Great job!
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