Nothing can be done on your iPhone. You need to re-configure your Router and allow better security options. Check your WiFi Router / Modem for --> Recommended settings for Wi-Fi routers and access points ...
Consider a 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
- TKIP is obsolete and not considered secure. Choose WPA2, the latest encryption standard, with AES encryption for optimal security
- If your router doesn’t specify TKIP or AES, the WPA2 option probably uses AES
See the pics below