My iPhone 5s isn't working the way it should?

So I got my iPhone on Dec 24. The people that worked at the T-Mobile store at the time said that there's a lag and it should fix itself within 2 hours, and that if it doesn't, take it back for a swap. It loses service and won't connect to Wi-Fi, I can't send/receive picture messages and I can't even websearch the issue because it won't connect. It says it's connected to T-Mobile LTE, but it still won't work… My dad told me that there's an app called onavo that causes this problem, and I don't have the app on my phone… I'm stuck. Please help me

Added (1). NoLedge: I don't even know what to google…

Take it to apple

Take it back to the store and have them repair/replace it. It only costs you your time.

Do a factory reset. If that doesn't work, exchange it. Google instructions if you don't know how

You are going to have to go to a T-mobile store. Having said this, I got
upset (mad) because I didn't think the counselor at the store knew what he was talking about. He told me to come back when I calmed down. I flew
home got on the computer, got an on-line person. She said I will not leave
until you understand what was done to fix the problem. This is true, 2hours
and 5 minutes later, the whole situation was settled to my satisfaction and the next day (sunday), this lady called me again to make sure everything was working. I'm a T-mobile fan. This just happened with in the past month.

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