Usual way to test the serial adaptor cable is OK is to short the tx/rx pins ( not easy with an RJ45 ) and enter some data in putty, you should see the data come straight back to you. You'd need to check the serial chip in mini pc and then see if there is a driver for it in FreeBSD.
cat /var/log/dmesg.today | grep uartuart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0uart0: console (115200,n,8,1)