Why does my moms old lg stlyo not show LTE internet connection when I use my sim card?

My mom had given me her old lg stylo, because she got a new phone and my old iphone 5s broke, so when she gave it to me I put in my sim card and I can't get LTE internet connection and I can't make any calls at all so what I did was I spoke with someone in T-Mobile(yes my sim card is t-mobile and also her phone) and they asked me for the IMEI and I told them and they had said that I need my moms old account to continue, but I couldnt do anything because my mom had switched to a different carrier and she doesn't have a T-Mobile account anymore so I was lost and then I was searching online in what to do and pretty much nothing helped there were things like restarting your phone, reset it, clean the sim card port, etc. But none worked so I'm now here today asking for any possible solutions, please help me.

She needs iPhone X

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