Here is the script for taking control of a device you purchase. I’m thinking the support team felt you wanted them to show you these steps or they don’t know how to remove MDM if they can’t touch the device physically due to SARS or just distance.
Take your time, there are a lot of linked articles and if this is your first experience this could take hours or on might need someone to help you with a loaner computer to restore if you don’t know all the passcodes and have a working WiFi and ability to join that network.
If that device is in their DEP, they can control that, so document what steps you performed and again, get someone else to help on the call if needed, that’s a negotiation where yo7 are trying to calmly convince the person / organization to do some work for you after the sale happened. There is no rush for them to take action, so be sure to let them know they have time to research DEP. you can even send them here is they don’t know how to release DEP. If needed, we will help them, too.