M402dne: no connectivity, is this thing junk now?

[ThoughtCube]

I have an M402dne that has been great for years. Unfortunately, I think it might be e-waste now. Recently, no device in the house could send a print job to it, though previously at least 5 devices had been sending successful jobs to it for a long time. 

On investigation, I found that it had a self-assigned IP address instead of getting an address in my home router's LAN range via DHCP, and was also set to AUTOIP instead of DHCP, which I thought was weird. My home router saw the MAC address of the printer, and had assigned it an IP address in the proper segment, but clearly the printer was ignoring that for some reason.

I power-cycled the printer, but this had no effect. Then I tried the following:

• Explicitly setting the printer to DHCP: did this multiple times- the settings were ignored, and the printer just went back to AUTOIP every time.
• Verified physical connectivity: link lights active on both printer and switch
• Changed ports on switch: no effect on behavior
• Set the printer to display its IP address on the console: did this so I wouldn't have to keep pushing console buttons or printing a configuration report to see what the printer thought its IP address was after every troubleshooting step. This was successful, but now I find I can't turn the feature off. The console acts like it's successful, but the IP address keeps being displayed, and when I go back into the settings, it shows the feature is enabled.
• Verified latest firmware: 20220414
• Did a factory reset (holding right-arrow and cancel buttons at printer boot)
• Used the restore defaults feature

Frustrated, I left it alone for a couple of weeks. Then I:

• Changed the IPv4 settings to manual
• Configured it with an address not used by any other device, and higher (.210) than my router seems to usually dole out (usually it gives out addresses with last octets from .20-.100ish
• Power-cycled printer again

No effect- but this time I got into the router and found that the MAC address of the printer isn't seen by the router. For the heck of it, I decided to disable IPv6. This was "interesting," as I found that now can't be turned off either, and I get weird behavior like the console dumping me back closer to the "root" of the menu than it should.

Tried really quickly to use it via USB to another Windows 10 computer and it seemed to set up correctly and printed a test page at the end of the Windows printer setup wizard, but then couldn't be found/printed to by that same computer immediately afterwards. Same thing happened when I downloaded and ran the "full" software package from HP for this model.

I deleted the printer from 2 devices- a Windows 10 desktop and an iOS device. When I attempt to add the printer, it is detected by both, but cannot be added/selected. Windows 10 just gives me a "that didn't work" error, and iOS grinds for a bit and then dumps me back to the print option screen with no printer selected. Other times it seems like it's ignoring configuration-changing button presses. Still reliably prints reports, for what it's worth.

At this point, my opinion is that the printer's control board (is that what it's called?) is irredeemably corrupt, and this printer is e-waste. But I'd love to try any other tricks anybody may have.