If you don't know the answer to that question, go to both PCs and load in a browser and compare the results. If they have different source IPs (PC1 and PC2 are either not NAT'd by the router, or get NAT'd to different IP addresses) you can filter administrative access by IP address. So the next question is: what source IP address does the router behind the 3G modem see PC1 and PC2 as having? Are they both the same? In that case you cannot meaningfully differentiate between the two and cannot permit access to one and categorically deny a login prompt to the other. Since you have the Internet between the two, that's more than one router.
How TCP/IP works makes that absolutely impossible.
If there is just one other router between the 3G modem and the machines administratively accessing the router behind the 3G modem, you CANNOT filter by MAC address.
