I'm trying to put an AT&t SIM card into an LG v20 T-Mobile phone
T-Mobile’s SIM unlock policy is pretty straight forward. If you meet the requirements, you can give Customer Care a call and request the permanent SIM unlock for the device.
SIM Unlock Policy | Unlock Your Mobile Wireless Device (t-mobile.com)
Enter your username or e-mail address. We'll send you an e-mail with instructions to reset your password.