This is a rather speculative response to this puzzling problem. Apologies in advance if it turns out to be a false lead.
Quite by chance I have come across a reference to "MAC spoofing", or in this case, "MAC cloning". It appears that, for whatever reason, ISPs may prevent - or discourage - connection from a device with a MAC address different from the one originally used when the connection was first set up.
One possible approach to this is to assume that this is being done as a measure for your security. On this basis, you might wish to contact your ISP; check that they do verify your MAC address; and ask them to change the address they use to the one assigned to your router.
A second possible approach is to assume - as I have seen suggested in print - that your ISP may be seeking to discourage the addition of a router to your network, by causing apparently inexpicable failures when it is used (this being detected by MAC address). Alternatively, there might be some other reason why the ISP cannot/will not change the registered MAC address. The remedy here is:
- find the MAC address used by the NIC on your "original" computer.
- Under "WAN Setup" on your SMC 2804 router, select the "Dynamic IP" menu item, and enter that MAC address as the "cloned" one for the router itself to use. (This gleaned from the user manual
here.)
This effectively fools the ISP into thinking that the connection remains direct from your original computer.
The easiest approach may well be to start with the second option, if you're happy with the technical details.
Again, this may be a total red herring, but probably worth investigating in the absence of alternatives.