I found the fixed ethernet had been assigned a static address when I upgraded mine, which I think was 192.168.0.100. You could plug it directly into a PC and try that after manually setting the PC IP.
If all else fails plug it into the router and if you know the MAC address add it to your arp table manually, then you should be able to get at it.
like this, use a free IP on your network
arp -s 192.168.1.2 01-02-03-04-05-06
you should then be able to go to that IP address, hopefully regaining access.