I've never seen a router with a serial port for input, although they were rather common to access modem settings when modems weren't yet equipped with a browser interface. But those were 'normal' 9 pins serial connectors, not USB.
The problem with serial was that you to specify if it was 7 or 8 bits, and if there was a start/stop-bot and possibly a parity bit. If those settings are wrong, it didn't work.
Using a USB-to-serial connection cable, and sending a file in stead of commands from the keyboard certainly complicates things. If I were you, I'd consider to use a router that has a management page that can be accessed from a browser using a standard Ethernet-connection.