Ive set up DuckDNS as the instructions here https://www.home-assistant.io/addons/duckdns/
and all works when I enter my domain name with the 8123 port number, but if I enter my domain name without a port number the webpage for my router comes up. Any idea how to correct this or what I’ve done wrong. The port forwarding rule was the IP address of my hassio with the public and private port set to 8123. Thanks
you did nothing wrong at all. duck dns forwards all requests to your router and if it happens to be a request on port 8123 your router then forwards it to hass.io. that’s as it is intended 
You will also find that if you are outside your Local Area Network (LAN), you will not be able to use the adress without port anymore. Thus you will not be able to get to your routers page anymore outside your LAN, since the port your router uses for its web interface has not been port forwarded.
Thanks guys for your quick answers, how would dnsmasq differ, would it be better to use?
Since everything resolves as expected this would not change a thing.
What is your intention? What would you like to achieve? Is it just about omitting the port?
I suppose I was being inquisitive really, heard about dnsmasq and wondered how it differed.
Duck DNS and dnsmasq serve two different purposes.
You can use Duck DNS to point one of its sub domains to an IP of your choice to allow easier connections to you own hardware without having a static ip address.
dnsmasq provides DNS service to small scale networks like your local network to resolve names which are not in the global DNS.
Thank you for your help