How can I get a more secure Wi-Fi connection?
There is nothing that can be done on an iPhone
Check your WiFi Router / Modem for --> Recommended settings for Wi-Fi routers and access points ...
Consider more secure wifi configuration.
Change your router’s WiFi to use WPA2-AES (CCMP) with no WPA or TKIP. Updating this setting ensures that your devices always connect in one encryption mode
See the pics below