There are a number of things that can affect entwork performance in circumstances such as yours.
a) is the connection at your apartment fibre or copper?
b) is each apartment assigned an IP from the ISP or are then NATed by the building's router and share an ISP IP?
c) is the building set up with VLAN's or some other networking architechture?
to your questions:
1) possible modem configuration issue, possible cabling issue, physical connection issue, switch or router issue (building or ISP) - you don't say whose tech support is stumped
2) unknown reason - don't look a gift horse in the mouth
no it doesn't use a different connection "type"
3) depends on the building. Each apartment may be a VLAN, be NATed, to a single IP, IP's passed through to apartments, etc. There are a number of options. Best place to find out is ask the building's IT person.
Sorry I wasn't more specific but they're general questions with lots of possible answers.