I was wondering what kind of uboot are you using in the gl-ar300m - I guess it contains custom NAND flash reader and writter. Can you put the tutorial how to compile it so that custom modifications can be made (e.g. serial port)?
Tried to look it up on your github, docs, etc… - This page does not contain information how to build uboot for gl-ar300m - Overview - GL.iNet Docs and GitHub - pepe2k/u-boot_mod: U-Boot 1.1.4 modification for routers does not seems to support gl-ar300m at all.
Can you please help?