I don't have a 3com router, but afaik it is unusual to have to mess around cloning MAC addresses when connecting via cable modem - most routers will work 'out of the box' with default settings.
- if it was me, I would reset everything back to default (obtain IP automatically, DHCP server enable, no MAC cloned) & try it again. (a full reboot of everything after resetting the router is probably a good idea, too)
hth
- aha, I see you have it sorted, 'alright folks don't touch that dial' - if it works don't try & fix it, but the above may help if you have any further problems