Eeps.... "I find out on the phone that the ACTUAL modem, DPX2213 isn't supported." getting this information from the ISP, and you want RedFlag users to over rule this...? There is a reason they told you it isn't supported, it might be a small incompatibility for an obscure function or just not listed in their material and cannot reliably support this modem. But more likely, just doesn't work as 'other models' from the same manufacturer are supported.
Quoting: "So I told them is a DPX2100 instead and gave the mac id / sn for it. Will that work?" Bet not... but you're almost too far down this road not to try.