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
DynDNS FAQ

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.
myonlineportal.net/checkip This one will always show you your ip your are currently using, can be IPv4 or IPv6.
ipv4.myonlineportal.net/checkip This one will only use IPv4. If you are behind a Carrier Grade Nat this can be not your public IP.
ipv6.myonlineportal.net/checkip 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)
DynDNS Update API

https://myonlineportal.net/updateddns?hostname=HOSTNAME&ip=IPV_4

Request Parameters - Header
Header Required Description
Authorization depends HTTP Basic authorization. A base64 encoded string of a username and password separated by a colon.
Not needed if username and password are query parameters

Request Parameters - Query
Parameter Required Description
hostname yes The dyndns hostname you like to update
ip recommended The new ip to update for the hostname, can be an IPv4 or an IPv6 address
Not required but recommended, if missing we will determine automatically your ip and update to the ip where the request was sended from.
myip   An alias for ip
ip6 no The new IPv6 address. Can be used in parallel with ip to update IPv4 and IPv6 together.
username depends Your username if you don't use Authorization header
password depends Your password if you don't use Authorization header

Responses
Satus Code Description
badauth 401 Invalid credentials, check username and password
nohost - Hostname missing 400 The hostname parameter is missing.
nohost - domain not found 404 hostname not found or not match to the user owned hostnames.
good IPv4_Address IPv6_Address 200 Update successfull for the given addresses.
nochg 200 IP's already set to the requested values, nothing changed.

Curl - Examples

Authorization header and hostname and ip as parameter

curl -u username:password https://myonlineportal.net/updateddns?hostname=example.myonlineportal.net&ip=10.10.10.10

Parallel update for IPv4 and IPv6 addresses

curl https://myonlineportal.net/updateddns?hostname=example.myonlineportal.net&ip=10.10.10.10?ip6=1ce:1ce:babe?username=USERNAME?password=PASSWORD


Compatiblity Issues

For compatiblity issues the update url will also work in following format

https://myonlineportal.net/dyndnsupdate.htm?hostname=...
https://myonlineportal.net/dyndnsupdate?hostname=...
https://myonlineportal.net/updatedyndns?hostname=...
https://myonlineportal.net/nic/update?hostname=...

ACME Update API

https://myonlineportal.net/set-acme?hostname=HOSTNAME&txt=

Request Parameters - Header
Header Required Description
Authorization depends HTTP Basic authorization. A base64 encoded string of a username and password separated by a colon.
Not needed if username and password are query parameters

Request Parameters - Query
Parameter Required Description
hostname yes The dyndns hostname where you like to set an acme txt record
txt yes The acme value you have to set, this can also be empty to clean up the txt record

Responses
Satus Code Description
badauth 401 Invalid credentials, check username and password
nohost - Hostname missing 400 The hostname parameter is missing.
nohost - domain not found 404 hostname not found or not match to the user owned hostnames.
set to 200 Update successfull for the given txt record.

Curl - Examples

Set an acme txt record

curl -u username:password https://myonlineportal.net/set-acme?hostname=example.myonlineportal.net&txt=sometxtvalue

Delete an acme txt record

curl -u username:password https://myonlineportal.net/set-acme?hostname=example.myonlineportal.net&txt=