Setting up dynamic DNS for linked router

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http://serverfault.com – I have a 'main' router that receives the internet signal from the ISP and another one in my room, connected with a cable. The main router is running its original firmware and is very limited in its features, unfortunately I can not change this router, since my phone company has some hardcoded stuff in there and the internet will only work with this router. My second router is running DD-WRT firmware. Now I need to set up dynamic DNS, so I can access my NAS machine remotely, which is connected to the second router. As mentioned, this can't be done with the main router, due to its limited feat (HowTos)