I have a netgear R6400 router that I'd like to flash with Tomato DD-WRT is there a way to flash it without connecting it to another router or internet someway to get to configuration page to be able to flash it
If I get it flashed I've got other questions to ask but that is a different part of the forum
You download them to a file on your PC. Maybe a router can get a image via internet direclty but I have always just loaded them from a PC.
As mentioned above do EXTREMELY careful research to get the exact correct image for your router. You can "brick" them pretty easy. Most routers you can now recover but it tends to be very tedious so you really want a correct image from the start.
Be aware not all routers can run third party firmware.