When building the kernel, the config file seems generated from some scripts. For example, for gt-ax6000, it will generate a config called "config_gt-ax6000". How can I change kernel configuration before building?
Okay, I think I figured it out. First, you need to find out the model number of your router's SOC. For example, GT-AX6000 uses BCM4912, then you need to edit config_base.6a.4912 to customise your kernel.