I want to know the department in MTN Nigeria that handles issue of Telecom mast and The Landlord

Hi Nnam, I think that it will be MTN Network engineers that will be in charge of Telecom Mast and other things related to it, you May need to get to the MTN regional office in your location to get in contact with them.

You can dial *123*7*1# to get information on the location of any their shops closer to you.
When you dial the code, you will be required to enter your State, then your Local Government Area, you will receive SMS on the nearest MTN office address in your location, or you can use this link: https://www.mtnonline.com/personal/find-a-store/