IPhone 12 Pro Max using WiFi 6?

The iPhone 11 and later is capable of connecting to Wi‑Fi 6 (802.11ax)

Now the question is what is the router/modem that you are connecting to? If it has WiFi 5 or WiFi 4 the iPhone 11 and higher will only connect to WiFi 5 or WiFi 4 because it has downward compatibility. If the router has WiFi 6 802.11ax it will seamlessly connect to it with WiFi 6.

In other words, if you have an iPhone, which has 802.11ac Wi‑Fi 5, it will connect to a router with only 802.11ac WiFi 5 even if the router has WiFi 6 (802.11ax).

Your iPhone 12 Pro Max supports the Wi‑Fi 6 (802.11ax).

The real question is: does your router support Wi-Fi 6 too?

If yes, your iPhone will use Wi-Fi 6. If no, your iPhone will use Wi-Fi 5 (or lower).