seacue,
If you're so confident that your ADSL modem with an ethernet port is a modem and not a router, I dare you to come back here and post the manufacturer and model number. I bet it's a router with embedded modem.
The institution somehow provides the Ethernet port on the wall. A fixed IP address is assigned. The router provides 4 Ethernet user ports and WiFi.
Now I am confused.
In your original description, it sounded like you were volunteering in a micro-company with a standard ADSL connection coming out of the wall.
Now you say that your 4 person team is part of some larger establishment ?
In which case, you deserve a slap on the wrist for plugging an ethernet switch into an ethernet wall socket at your own accord without consulting site techies.
Plugging in a cheap unmanaged switch into a managed network has been known to cause routing loops and spanning tree topology issues (i.e. no physical damage, but technical chaos !). But it sounds like you've lived to tell the tale because the admins configured their side with some precautionary settings anticipating experimental users.
Or does the organisation just use structured cabling and provide you with a phone socket on the wall through a balun ?