That imei doesn't show in T-Mobile imei tool. Are you sure it is locked to them? Usually that means T-Mobile didn't sell it.
If it is a T-Mobile lock and being you are not a customer, you will not be able to get it unlocked by T-Mobile. You will need to find a site online that you can purchase an unlock from.
Serial No. F4GS8ENKHG6W
Activation Policy Details
Operation Id: CY3uRyBjVHqqlxMLYrfS3
Applied Activation Details: US T-Mobile Locked Activation Policy
Applied Activation PolicyID: 4000
Bluetooth Mac Address:
First Unbrick Date: 18/09/16
IMEI Number: 359168070308731
Initial Activation Policy Details: US Reseller Flex Policy
Initial Activation Policy ID
Last Restore Date:
Last Unbrick Date :
Mac Address :
MEID : 35916807030873
Next Tether Policy Details : US T-Mobile Locked Activation Policy
Next Tether Policy ID : 2136
Part Description: iPhone 7
Product Version: 10.3
Unbricked : false
Unlocked : false
I thought the seller can call t-mobile support and he will be able to sort this out. And he said the phone is now yours and this is your problem.
If the seller is unwilling then there is nothing T-Mobile can do. The original owner is the only one that can work with T-Mobile to get it unlocked through them. Unless the seller can see if they meet the requirements for unlock and work with T-Mobile to do so, your only option would be to find a service online to buy the unlock from.