how to remove a carrier sim card lock, when the carrier is unable to

There are several things here:

• Carrier Lock ties your phone to the carrier and the carrier would have to remove it before you could change carriers.
• SIM card lock happens when you have the SIM Pin setting turned on and enter the wrong SIM code too many times. When this happens the phone will ask for the PUK code. Only the carrier can provide the PUK code. If you enter the PUK code wrong too many time, you will need to replace the SIM card.
• Activation Lock will appear on your screen if the original owner reported it as lost and will be of no use to you.

That is a different issue altogether, but the solutions are the same. Only T-Mobile will be able to resolve this, you should try to take the device to a local store and see if they would be able to escalate the problem to tech support that may be able to help. They may not be able to solve it at store level, but may know the path to get it resolved quicker than just playing phone tag or using online support.

Only the Carrier to which the Carrier Lock exists can remove that. Not even Apple can remove it.

You don’t. Only the carrier the phone is locked to can unlock it for use on other networks.