Michel just saved the day with the following solution.
Step 1. manually get IPv4 networking up and running again - although he gave me the shortcut of running just one command to do it all: "sudo dhclient re0"
Step 2. run their fix.upgrade shell script via "curl -s https://pkg.isy.io/script/fix.upgrade | sudo bash"
That triggered a few reboots and then things were back up and running under 5.9.1 normally.
Thanks Michel!!
