I followed this guide from that step is enough 5 DDNS Client Software - Linux DDclient (step 5 in content) I edited ddclient.conf this way
use=vigor-2200usb, fw=192.168.1.1:80
fw-login=user, fw-password=yourrouter_pass
login= my_username_for_freedns.afraid.org # default login
password=my_pass_for_freedns.afraid.org
protocol=freedns
#proxy=fasthttp.sympatico.ca:80 # default proxy
server=freedns.afraid.org #
#server=members.dyndns.org:8245 # default server (bypassing proxies)
robertobalu.com.crabdance.com # domains must go after those login, use staff
robertobalu.com.chickenkiller.com
#mx=r.balejik@robertobalu.com.chickenkiller.com
you’ve to put it in that order otherwise you’ll get message like invalid login …