While we might be able to help you install OpenWrt in your device, there are people on the DD-WRT boards that can help you with that firmware.
It’s a pretty dated device, really too under-resourced these days for current, secure Linux. I don’t know your budget, but you might want to consider replacing it with one of the GL.iNet units. I have both the GL-AR300M and the GL-AR750S. There is a less expensive GL-MT300v2 as well.
Edit: There are many other suitable devices (16 MB or more of flash, 128 MB or more of RAM) starting at around US$20. The two that I mentioned are because I can recommend them from personal experience as having excellent design, build quality, and OpenWrt support.
Any Linux-based firmware older than a couple of days at this point has known, critical vulnerabilities that can be remotely exploited. When you choose a firmware for your device, how quickly the team responds to issues like this is important to many.