I have managed to get a root shell on my web filtering router by plugging into a UART on the PCB which allowed me to see a process (lockbox.bin - ELF executable) running which handles most of the filtering functions. After extracting the lockbox.bin file and analyzing it with IDA pro, I've made some modifications to the file. I now need to get it back on the device to see if the changes worked, but when I try to make changes to the filesystem I get a message saying that the file system is read only. I suppose it's only a matter of mounting the filesystem as read/write, but I'm not sure how to do that (Access the boot loader?). The Busybox instance on the device is missing the mount command. The router in question is a re-branded realtek rtl8196b.
Disclaimer: This is the first time that I've have done something like this so I may be missing a very basic step. Any help would be appreciated.