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To access config file, you should use terminal to login into router’s shell by SSH protocol.
Pls refer this to use ssh,

After login into router’s shell,
you can issue some command. like:
installing a editor named “nano”:

opkg update
opkg install nano

You need internet access here.

Then nano can open config file and you edit it:

nano /etc/configfile_name

Another editor installed by default is “vi”, that’s a little bit complex, but more prevalent on router system.