DD-WRT includes a Dynamic DNS client with a Custom option that works with any DynDNS2 provider. With DNS Beacon your router keeps your network reachable under a permanent hostname even when your IP changes. Here's how to configure it.
Services → DDNS
Set the DDNS Service to Custom. DD-WRT appends your hostname and current IP to the URL automatically.
Create a free DNS Beacon account, add a hostname and generate an API key — then come back and finish the setup above.
Create free account