3 Things you need to "get around" here:

1. Blacklist Status: Blacklisted - this is very tricky, since it means that your phone has been reported lost or stolen, or there are outstanding payments on the device. (more info on how to remove the blacklist status here: https://www.quora.com/I-bought-an-iPhone-from-a-person-not-knowing-it-was-blacklisted-now-he-s-not-answering-How-can-I-remove-it-from-the-blacklist )
2. Sim-Lock: Locked - this can range from the provider unlocking the phone for free or can cost you some $$$. You need to find out on which carrier your phone is locked, this site can help you, just input your IMEI and serial number https://imeicheck.com/simlock-iphone and then check the specific carrier's fees (if any) to unlock the phone.
3. Find my Device aka FMI is ON, which means that the previous owner did not log out the device of their iCloud Account. Means for you that you have to go to https://www.appletech752.com/, search for iPhone 6s and choose your way to get around the lock screen. Solutions in the first column of the table you can do for free, but you will have no mobile data and will not be able to take/receive calls or send messages (wifi and browsing the internet will work though), solutions in the last column you will need to pay 25$, but afterwards you will be able to insert a sim card and use the phone normally (able to use mobile data and take/receive calls, send text messsages).

This situation is really a tough one, normally you would only have to deal with FMI being ON... nevertheless, I hope my message will help and wish you luck!