I need to use a sofware that only supports IPv4 on iOS 15, but my ISP provides IPv6 addresses by default.
How do I force my iOS device to use IPv4 only on a certain Wi-Fi network, or the other way around (force IPv6)?
When an iOS device connects to a WiFi-network, it will by default request both IPv4 and IPv6 addresses. If you only see an IPv6 in the settings, it is because your ISP only gives out IPv6 addresses.
If you want to use IPv4 on that WiFi-network, you will need to either get your ISP to change their settings to enable IPv4, or use a VPN service that you can connect to over IPv6 that gives you a virtual IPv4 interface.
It is not possible to disable IPv6 for a specific WiFi-network or for WiFi-networks in general on iOS. Similarly you cannot "force" IPv6.