yum install ddclient
vim /etc/ddclient.conf

######################################################################
 21 daemon=600                              # check every 10 mins -> 600 seconds
 22 syslog=yes                              # log update msgs to syslog
 23 #mail=root                              # mail all msgs to root
 24 mail-failure=root                       # mail failed update msgs to root
 25 pid=/var/run/ddclient/ddclient.pid              # record PID in file.
 26 ssl=yes                                 # use ssl-support.  Works with ssl-library

 47  fw-login=admin,             fw-password=theRouterAdminPassword                # FW login and password

 58 #use=web, web=checkip.dyndns.org/, web-skip='IP Address' # this works
 59 use=web, web=ip.changeip.com/, web-skip='IP Address' # but this uses the changeip resources so is more polite

188 ## single host update
189 protocol=changeip,                   \
190 login=myChangeIPUserName,      \
191 password=myChangeIPPassword    \
192 #HereWouldBeTheDomainWeAreUsing.com # but it's commented out because we are applying the IP to the WHOLE of a set
# that is to say we want to change mail, www and ftp etc all at the same time
# Nominate the Set in the Chanegip website in your dns settings # for changeip this can be set1 or set2
# there are 'tick' boxes on the right!
193 *1 # and leave this uncommented so that it applies the change to the whole of set 1
194 #*2 not used by me at this time
194

Test in a terminal with:
ddclient -daemon=0 -debug -verbose -noquiet

 vim /etc/sysconfig/ddclient
 remove or comment out
 # DDCLIENT_OPTIONS="-daemon 300"

 revised systemd service file is as follows:
 vim /usr/lib/systemd/system/ddclient.service
 1 [Unit]
  2 Description=A Perl Client Used To Update Dynamic DNS
  3 After=network.target
  4
  5 [Service]
  6 User=ddclient
  7 Group=ddclient
  8 Type=forking
  9 PIDFile=/var/run/ddclient/ddclient.pid
 10 ExecStart=/usr/sbin/ddclient -file /etc/ddclient.conf
 11
 12 [Install]
 13 WantedBy=multi-user.target

# the cache is installed incrrectly owned by root
# correct to enable use by ddclient
chown ddclient:ddclient /var/cache/ddclient/ddclient.cache

 systemctl enable ddclient
 systemctl start ddclient