Ran the upgrade from Fedora 30 -> 31 per the usual dnf process. Everything went well EXCEPT: after restarting and signing in my ethernet port on DHCP (Intel 218V gigabit) was configured only with IPv6. Forced an IPv4 address onto the i/f then set routing to the ISP router, but problems continued: couldn't see my NAS box (connect failed), couldn't pull up a browser to any external location. Thankfully my mobo has dual NICs, and moving to the secondary jack (Realtek RTL8111GR) everything came up fine. Anyone have a similar situation show up, or a suggestion for recovering the original NIC?

setup: Asrock Z97 Extreme 6, i7-4790k, AT&T fiber router (Arris/Pace 5268AC).

The NIC's hardware was recognized, otherwise it wouldn't get an IP address at all. To troubleshoot DHCP, first ensure there is a physical connection to the DHCP server. I think you have done that already.

Once this is established, check logs on the server to see if it receives requests. You can also trace ports 67 and 68 on the client. Try obtaining an address by running dhclient manually (not sure though if F31 uses dhclient).

If that works, I would guess that the update to F31 somehow messed with your NIC config. Check /etc/sysconfig/network-scripts/ifcfg*, and use nmcli to confirm how this NIC is configured. The nmcli con and nmcli dev commands should give you first information.

Of course, this assumes that F31 uses the same network config as earlier releases.