I doubt that they are actually using MAC filtering, but probably some kind of captive portal that the Remarkable doesn't have support for (without setting up a SOCKS proxy, which doesn't count as a solution, IMO)[0]. Most of these captive portal systems allow someone with privileges to add a MAC address to the system manually, so that device has access without having to go through the portal itself. In the old days, that was the kosher way to get Xboxes and the like on hotel Wi-Fi (they'd often have a number you could call from the room to get a network admin to put your widget on the list).
0: https://remarkablewiki.com/tips/wifi
(Also, given that OP's a part time helpdesk person, they can't turn the MAC filtering off anyway).