The commands I needed to run to allow SSH to a headless Debian 12 Pi OS install were:

mkdir /mnt/pi
mount /dev/sdc1 /mnt/pi
touch /mnt/pi/ssh
echo pi:$6$fGzsGCnOaRjJxAl7$oyr66oRzD4e5jIKmJgE7Xxnvtg...

I have a Draytek Vigor 2762 which has been working well with my block of 8 static IPv4 addresses for a few years, but now my IP range is changing so I needed to update the router settings. It's still a /29 but different addresses.
When I first got the router it was easy to click on IP Routed Subne...