After using all my high-speed data, I still got 3g coverage

Someone has put something on my phone and I believe it's my ex but I can't prove it. I ran out of data and even thought to myself 2.5g isn't slow at all but then my phone cut off (needed to pay the bill) I could only go to boost mobile website and while trying to add money I got to see how slow 2.5g is. My question is what would he being doing so that I still have high speed data. I believe he is accessing my phone through his computer whether it be an shh client or something similar. Please help BC idk enough to even really know what I'm looking for and I think it's possible that he hacked my phone BC I signed into his wifi network. He is a developer and I'm far from me it. Also I put an old SD card in my new phone and note there are files saved on my phone I cannot delete do maybe that's how he does it

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