please, tell me what your skills are with Linux (what are your previous Linux usages? Do you use Linux sometimes/daily?). I need it to give you adapted answers. Because I answered a lot about your questions/problems, and the progress in the solves are long.
Also, you should be more verbose in your questions and answers (give the command you tried, the output results of the commands, the log traces), else this ping/pong discussion will be very long.
Finally, I also need your read the Qubes OS documentation, else you will progress with difficulties. The Qubes OS documentation is very well done.
Me, I will also try to give you more detailed answers. Say me if I should improve something else in my answers.
The Qubes OS default kernel (4.9) is too old to recognize the I219-V (8086:0d4f) internal Ethernet PCI NIC. So you should launch the below command  in dom0. BUT this command need network access, so you see it seems to be an unsolvable solution…
So, one solution is using another ethernet NIC which is recognized by the Qubes OS default kernel, configure it, getting network access and so update the Qubes OS kernel with the . Me I used a USB NIC, as described in my mailing-list post. You are right, it should be possible to get the new kernel and transfer it to dom0, but I haven’t try this, you can try it if you know what you do and how to do it properly. Another solution is the internal wifi card, but I can help you with it, I don’t use wifi on Qubes OS. Another solution is a USB wifi adapter. … It depends the hardware you get and your skills.
You don’t say me if you got a USB NIC and plug it. Please explain me more, give me the command outputs, give me traces. Also you should understand the commands.
For example, the traces (in sys-usb) when I plug my USB ethernet NIC:
[user@sys-usb ~]$ sudo dmesg -Tw
[Mon Nov 16 21:06:16 2020] usb 3-3: new SuperSpeed Gen 1 USB device number 2 using xhci_hcd
[Mon Nov 16 21:06:16 2020] usb 3-3: New USB device found, idVendor=0b95, idProduct=1790, bcdDevice= 1.00
[Mon Nov 16 21:06:16 2020] usb 3-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[Mon Nov 16 21:06:16 2020] usb 3-3: Product: AX88179
[Mon Nov 16 21:06:16 2020] usb 3-3: Manufacturer: ASIX Elec. Corp.
[Mon Nov 16 21:06:16 2020] usb 3-3: SerialNumber: 000000000009FF
[Mon Nov 16 21:06:17 2020] ax88179_178a 3-3:1.0 eth0: register 'ax88179_178a' at usb-0000:00:06.0-3, ASIX AX88179 USB 3.0 Gigabit Ethernet, 00:23:56:11:22:33
[Mon Nov 16 21:06:17 2020] usbcore: registered new interface driver ax88179_178a
[Mon Nov 16 21:06:17 2020] ax88179_178a 3-3:1.0 ens6f0u3: renamed from eth0
[user@sys-usb ~]$ lsusb
Bus 003 Device 002: ID 0b95:1790 ASIX Electronics Corp. AX88179 Gigabit Ethernet
And then, this device appears in my Qubes device manager.
Edit: I haven’t seen your Edit:
No you should attach it to sys-net, not sys-usb. Re-read steps #4, #5, #6.