Slave device is probably the wrong concept (it's also on the word replacement list of the INI, but that's a sidenote).
It'd be a regular host in the ISP router's network. If there are any settings in the ISP router to prioritize devices, I'd probably enable those for the DD-WRT router as well. Should be pretty much plug and play otherwise, besides the obvious ordeal of configuring DD-WRT just right, of course.