If you have a dynamic public IP, you'll have to set up DDNS since a normal DNS only points to an ip statically, and if your ip changes, everything goes down until you update the DNS record.


DDNS does this automatically. 

select a reliable service:
noip.com
duckdns.org 
dyn.com 
dynv6.com 

follow their install instructions. Generally, your router will have DDNS support. 

If it does not, you can install a DDNS client on your server (apt install ddclient)