How do I make my network more secure? It doesn’t ask for a password to join

Your WiFi network won't ask you to enter a password on your iPhone word every time because your phone remembers the password and sends it automatically. You can go to Settings>WiFi and delete your network from Settings when you're done using it but you'd have to do that every time. And, doing so wouldn't make the network any more secure.

The router needs to be configured properly. There is nothing on the iPhone that can be done.

You might find this article helpful when configuring your router:

Recommended settings for Wi-Fi routers and access points - Apple Support