Your ISP keeps changing the IP address from you. With DynDNS you can bind this IP to a firm and easy to remember web address. Now you can reach your home router and other network devices like NAS from the outside.
-- wikipedia

CheckIP Script
If you need an easy way to detect your public ip address, we have a solution.
We provide several url's to detect your public ip. This one will always show you your ip your are currently using, can be IPv4 or IPv6. This one will only use IPv4. If you are behind a Carrier Grade Nat this can be not your public IP. This one will only use IPv6 if available. Otherwise you will not reach this url.

Lets Encrypt Wildcard SSL Certificates with acme-challenge
If you enable wildcard for your domain, you will also see a textfield for entries for _acme-challenge txt records. These records can be set during the certbot creation or renewal of the certificate.
More details are available here -> Lets Encrypt DNS-01 challenge
DynDNS HowTo's

Fritz!Box (Router)
OpenWrt (Router)
ddclient (Linux, Mac)
inadyn (Linux, Mac)
DynDNS Service (Windows)