How do I know if my iPhone device is using WiFi 6
In a nutshell, WiFi 6 is a marketing term for 802.11ax and the fastest way to know it’s working is to check radio details on the Ubiquity app and know exactly which encoding scheme is used from moment to moment. The radios adapt and steer traffic based on interference and other radios which change constantly.

https://en.wikipedia.org/wiki/IEEE_802.11ax

Even with WiFi 6 available, if you’re stuck behind a slow router, a well engineered device might connect in a more power-efficient manner until it needs extra speed.
Both Apple and Ubiquity have a long track record of working together well -  in the end, I recommend you look at the WiFi quality on Ubuiquity side and check your iOS device MAC address in the UniFi App and check your transmit rate / channel / RSSI to make sure your speeds are negotiating to the fast speeds you expect. As long as WiFi is faster than your server connection, it likely won’t matter if you’re on 5 or 6 flavor of the technical / marketing side of things.
To illustrate this recommendation:

I currently have a 802.11ac connection so the 866 MBps means I’m either running MCS 9 with two spatial streams and 256-QAM modulation on 80 MHz channels with 400ns GI or MCS 9 with one spatial stream on a 160 MHz channel and the other radio parameters the same. Since WiFi 6 includes older standards, you could argue I’m already using a WiFi 6 connection with my hardware for this specific answer.