Have you tried connecting the device after your router such as in this configuration to give the Mini a different address:
Modem -> Router -> Mini -> PC
Either case should work as the Mini uses the MAC of whatever device comes after. However, from your description it sounds like you do not have access to the MAC authorization and you have to specify an allowed address? Maybe your provider does not allow duplicate MAC addresses and that is a part of the problem. If you have any controls over the MAC authorization or are able to contact your provider about this then maybe they can work with you to allow the device to function. I believe setting the MAC address manually would cause issues with the functionality of the device but I can inquire about this with the dev team.