When flashing an OpenWrt image to your newer versioned WRT1900AC/S, be aware that instead of using ‘setenv firmware_name’, you should instead use ‘setenv firmwareName’.
The command will not fail, but the router will not understand that it should look for a non-default name for the image and your tftp transfer will fail.
This change appears to have been made between WRT1900AC V1 and WRT1900AC V2. So, for the latest versions such as WRT1900ACS, be sure to use ‘setenv firmwareName’.