This right here is the very reason OpenWRT exists.
I never trust ISP provided equipment to do my routing, if I can't use it in modem mode (or provide my own modem) then a DMZ and port forwarding have to do ... but I'd sooner just choose another provider.
At least you need to know the settings for this. I tried attaching my laptop directly to the cable and that didn't work (I would put my own router if it did). There probably is some sort of PPPoE over a statically-configured network.
I never trust ISP provided equipment to do my routing, if I can't use it in modem mode (or provide my own modem) then a DMZ and port forwarding have to do ... but I'd sooner just choose another provider.