Reboot the PC and tap the Dell F2 key to access BIOS setup. Browse carefully through each tab to see if you can see LAN settings for the ethernet connection which have been changed. Then reset BIOS to Defaults, save changes and exit.
Does the ethernet adapter show up in Device Manager now? Try on the View tab enabling Hidden Devices.
Next enable wireless driver
and run Windows Updates repeatedly with reboots to install all IMportant and Optional Updates in the hope it will provide the missing driver.
In addition plug in the ethernet cable and watch closely for activity in the bottom-right System Tray. Then unplug the router and model and plug each back in. Try plugging ethernet to both the router and the modem while watching for activity on SysTray.
Do you have a phone plugged in that's needing those CDC drivers
? Have you checked the Dell Support Downloads webpage for your tag number to see if there are drivers for that, or tried another LAN driver provided? I would reinstall the chipset and then try all LAN drivers.