There is no such thing as perfect security. Given enough knowledge, resources, and time any system can be compromised. The best you can do is to make it as difficult for an attacker as possible. That said there are steps you can take to harden your network against the vast majority of attacks.
The default configurations for what I call consumer-grade routers offer fairly basic security. To be honest, it doesn’t take much to compromise them. When I install a new router (or reset an existing), I rarely use the ‘setup wizards’. I go through and configure everything exactly how I want it. Unless there is a good reason, I usually don’t leave it as default.
I cannot tell you the exact settings you need to change. Every router’s admin page is different; even router from the same manufacturer. Depending on the specific router, there may be settings you can’t change. For many of these settings, you will need to access the advanced configuration section of the admin page.