Any question answered...

Certainly no expert here, but maybe can give you some ideas..

Look at the tcpdump command, you can see what sort of packets the device is sending when you connect it. The arp (apr -a) command might also be handy.

You can start a dhcp server to see if you can assign it an ip, which will give you some indiction of what is supposed to do.. alternatively, connect it to your router - you can then log into that and see if it is assigned an ip. If you can assign an ip to it, you can then do a portscan and hopefully telnet, though you might need a password!

Some good ideas thanks.

but it might be a serial port, rj45 jack is not necessarily just for tcp/ip..

I was not aware of that! Hmm I will see what happens, need to grab a cross over cable as router is too far away physically, although I might have an old router somewhere.