Correct, a SFP module would obsolete the ISP’s modem. That said there are various reports of SFP modules currently not working properly with TOS4.x and later.
If you want to read about it there is a thread in the forum [1]
What sort of codes are those? →
For that purpose that ISP’s modem would be in a full/half bridge mode and let the TO handle the authentication.
Probably indicates that the modem is just a modem indeed without any routing/dhcp/firewall capabilities. Do you have the specs of the modem somewhere online available?