When I sign into my Wi-Fi, it says that I have a low security connection. How do I fix that on my iPhone 11

How do I fix that on my iPhone 11??

You don't fix that on the iPhone, you fix it on the Wi-Fi router. The iPhone is capable of handling higher security than your Wi-Fi router is currently providing.

Ideally, your router would be set up for WPA3 security, but WPA3/WPA2 would be OK. If neither of those settings are available, choose WPA2 (PSK) AES.

You might find this useful:

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