I'm really confused ... RJ45 ports on laptops / PCs are for Ethernet connections, right? I do not understand how you could use it as a COM port.
If you need a serial port and do not need ultra-low latency *, take the USB ↔ RS232 converter, they will work from 20 to 30 US dollars and will work perfectly: from the point of view of PC software, it looks like a normal old COM port.
* RS232 , USB , , USB... , , USB 1 16 ; USB 2.0, , 125usec, , - . , RS232 .