I have an ASUS that I could I guess, but I'm not sure why you would need custom firmware?
I think that Custom firmware like DD-WRT is usually more secure than most router firmware's which become abandoned by the router makers after a few years, so firmware from router makers may have unfixed security problems, or the router firmware may use unsafe firmware from previous version of the firmware which they used on their older firmware.
Custom firmware also sometimes have more features, and better reliability than firmware made by a router maker because the custom firmware maker has staff which are more experienced at coding good firmware which can be difficult.
Custom Firmware may have better privacy as well because custom firmware is less likely to include spyware, bloatware, and adware which can spy on your browsing habits, and monitor your router usage. I think the US Government banned Chinese made routers from government buildings because they feared that the Chinese router has firmware/software which can be used to spy on the US's internet traffic.
I bet, most big governments, and companies probably use custom firmware on their router's to protect their network traffic, and browsing history.