What would happen if I send 220V to my ethernet port?
Ethernet ports typically have an isolation transformer inside them that have an isolation rating of a few kV.
Depending on how the cable is wired, it will either do nothing, or it will destroy one side of the transformer wiring.
Other than burning out the transformer, the rest of the computer would likely be unharmed.
The transformer is likely to be the thing that burns out first rather than the actual cable because it probably has smaller gauge wires, and they are all packed together, which is much worse from a thermal perspective.