If an intruder gained access to a network impersonating the mac adress and the device name to appear like a familiar device in the network, how then is it possible to detect him in the router logs ? can the router see his card manufacturer or other informations that hackers can't change ? I know completely spoofing a device identity is impossible, correct me if i am wrong.
MAC addresses, IP addresses, and DHCP hostnames are not security controls. All of those can be forged. I can also make my TCP/IP stack look like any operating system available -- the exact same signatures that tools like
p0f use can be used to configure my network stack.
A strong WPA2 passphrase is the only effective wireless network security control most consumer routers offer. (And since you are using the term "router" for a device that your client connects to, I assume you mean a consumer wireless router.)
There is generally nothing logged on a consumer router that would allow you to detect a malicious device masquerading as a legitimate device.
You can try to identify operating system using packet fingerprinting.
For example, if your normal host is Windows, and attacker is using Linux, you can detect OS change. The only condition is that the remote host sends at least one packet during fingerprinting process.
You can perform such fingerprinting with nmap, for example:
nmap -O ipaddr