iPhone was pre hacked to always have my wifi and Bluetooth on please help!!

I bought my iPhone XS off of Facebook from somebody I know that my stepbrother knew we don’t see eye to eye so he pre-hacked my phone before I bought it believe it or not and I know that it is a hack that has my Wi-Fi on all day and my Bluetooth which I don’t want to have on when I’m home because all he does is stream my phone on his tv or computer next door and I know this for a fact… how do I get rid of this? The police aren’t really doing JACK about it either it’s ridiculous and I have no privacy… pls help!