Dynamic DNS (DDNS) is a service that maps a static hostname (for example, mysite.dyndns.org) to an IP address that changes over time. The DDNS client running on the router detects when its public IP has changed and updates the DDNS provider with the new address, so the hostname always resolves to the current IP.

In short: DDNS allows remote access to a device on a connection where the public IP is not fixed. Many cellular IoT SIMs without a Fixed IP service give the router a dynamic IP from a pool, and that IP can change with every reconnection. DDNS hides that change behind a stable name. Industrial routers from Teltonika and others support DDNS clients for major providers including No-IP, DynDNS, DuckDNS, and Cloudflare.

DDNS has obvious limitations for production IoT. The hostname is publicly resolvable, which means anyone who knows it can attempt to connect, and the underlying IP changes do not protect against scanning. For low-stakes remote access (a small business owner wanting to view their own CCTV camera) this is acceptable. For commercial IoT estates, Fixed IP SIMs, private APN with private IP addressing, and outbound-tunnel platforms like Teltonika RMS are all stronger architectures.

That said, DDNS remains useful as a low-cost option for development, testing, and small deployments where the alternatives are not commercially proportionate. It also has a place as a fallback path in some VPN configurations.