I'm afraid there's no way to do this. I contacted my ISP who said that the modem (model ET2251) is already in bridge mode and there's no way to access a DMZ function because there's no way to adjust the modem settings.
My ISP is Spectrum and I have the R3. I can't seem to configure an open NAT for black ops 6. I'm using a wired connection. Disabling firewalls didn't do anything.