Does your modem hand out DHCP or did you forget to setup a PPPoE-Client on the Mikrotik?
Yes i believe so, i did manage to fix the problem by unplugging the modem. Not sure why a modem would require MAC addresses to match but it does.
So the cable companies can charge you for additional IP addresses.
The modem will only allow the first MAC address seen to get an IP address. It doesn't matter what it is .. just the first. That's why a new device with a different MAC requires a modem reboot, to clear out the old one. Or, if you clone, then the new device looks just like the old one ... and no modem reboot is required.