LaserJet won't properly connect to network

[DASPRiD]

I've been doing so much debugging on this without much success, so I hope someone here will be able to help:

Since over a year I now have issues with the networking of my MFP printer.  From practically one day to the other (after I factory-reset my router), my printer stopped connecting to the network. I did some investigation with Wireshark, and it did list some (according to WS) broken DHCP requests, but no responses from the router. After a while the printer falls back to it's fallback IP address in the 255.255.0.0 subnet, though even if I set another device to the same subnet, I'm not able to reach it.

No big problem I thought, let's just set the IPv4 config on the printer manually with the correct subnet and gateway. Well, this looked promising, as now the printer showed up in the connected devices on my router web interface. Even though that was the case, I was still unable to reach it under the fixed IP address. I also tried multiple times to factory-reset the printer, without any results.

At this point I was wondering, whether the network interface of the printer was defect. The next thing I tried was to connect the printer directly to the second ethernet port of my PC and assigned a fixed IP address to set up a direct link. Lo and behold, now I was able to reach the printer. I downloaded the firmware update utility and applied the latest firmware to the printer, in the hopes that it would solve the issue (which to this date is still the most recent one). Sadly this didn't improve the situation, as DHCP was still failing and a manual IP on the router was also still not working.

I've been using the printer for over a year now on my second ethernet port, which was fine to me. The issue is that I'm upgrading my PC  now, and my new motherboard does not have a second ethernet port.

Thus I'm asking if someone might have ideas on how to fix the issue, if it is not a firmware or hardware fault. If it is one of the latter, I might consider getting an ethernet-to-wifi dongle to work around the issue in the future.

Before anyone asks: The reason I'm not using the USB connection is because I'm making heavy use of scanning directly to a network drive.

[Repairatrooper]

Have you tried modifying the connection speed of the network port on the printer. I have found some routers have issues connecting when the printer is set in Auto-negotiate and setting it to 100 full duplex can sometimes fix the problem.

[DASPRiD]

I have in fact tried that yesterday, and went through different speed settings, none of them yielded any improvement.