A good internet service provider (ISP) will should dynamically update their DNS records but some may not. If you do not know if your ISP caches DNS records or not we recommend that you use DNS name servers that are known to refresh quickly / dynamically, not by caching records.
We recommended using DYN's DNS / name servers:
Please also ensure you have referred to DYN's Internet Setup Guide.
Importantly, in addition please ensure that your local devices (firewalls, gateways) do not cache DNS settings.