I've been using the method described here:

https://www.dynu.com/DynamicDNS/IPUpdateClient/RaspberryPi-Dynamic-DNS

For several years. Suddenly it has stopped working, but I have no idea why. Has something changed?

Here is the code that has been working:

echo url="https://api.dynu.com/nic/update?username=mine&password=mine&myip=$ip" | curl -s -k -o $dynfile -K