A long long time ago, I use to run DD-WRT on my router. And then I ran AdvancedTomato.
This was my first, and my last, experimentation with custom firmware on routers.
At the start of 2014, I decided to put DD-WRT on my Asus RT-AC68R. It ran horribly. Although I enjoyed the additional customization, the router locked up every 3-7 days and had to be rebooted.
After reverting back to stock firmware, I then put AdvancedTomato on my router, which is based on the Tomato firmware. This actually worked pretty well, was stable, had a good GUI, but did have its flaws. It ran on my router until it was replaced.
Since 99% of this page is now gone, I’ll leave you with one tip: If you’re trying to put a custom firmware on a router, just get enterprise-grade equipment (aka Ubiquiti). You won’t be disappointed.