Oh, now I see.

Code:

/dev/root on / type squashfs (ro,relatime)

You aren't going to be able to write to this. It's an unwritable compressed filesystem which is stuck the way it's made.

You'll have to copy it elsewhere, extract it all, edit it, and create a new squashfs to replace it with.

If you don't get it exactly, perfectly right, things may go badly. I suggest finding newer firmware instead of hand-editing things yourself.