It's an SMC D3GN from Rogers; we have configuration access to it (and the .pdf manual).
Unfortunately, it appears the only way to bypass the NATing is to define a DMZ address on the SMC and then make the RED (external) address on the ASG that DMZ address.
We can't disable port forwarding.
I will ask about getting the ISP to bridge it, but I'm not optomistic.
Will Rogers let you use your own modem if you buy one?
Amazon, et al. have Motorola SB6120 'SURFboard' DOCSIS 3.0 modems for under $100USD.
Some of the cable companies here actually give a few dollars off on the monthly rate if you have your own modem, but you have to give them the serial # to set it up, and again if they change their configuration (rare).