Setting WEBHOOK_URL is what you would need to do but how you would do that would depend on how you have installed n8n, n8n would also need to be restarted after this change has been made. When you do set it though make sure you include the protocol as well (http://192.168.x.x).
Did you try uppercase? I probably wouldn’t follow those steps as they look to be a bit out of date and have a potential issue that could cause a break. I tested setting a webhook last Thursday for another issue and it was working on Ubuntu so it would be handy to see the exact command you are running.