What's your router? Have you tried doing a software update, or maybe just rebooting it? Lots of folks in the below thread are reporting 6Ghz support, but some have had issues at first though eventually they got it working:
Got the 12.9” today. Connects without issues to my eero 6E mesh network on the 6GHz channel. Speeds are about 30% greater than my 5GHz devices get, as there is no other traffic on the 6GHz channel right now.
It works great. With my RBR960, all bands under one SSID, M2 iPad picks up the 6ghz band with the speeds that go with it. A separate 6ghz band would give you “Limited Capability“
The new ipad pro supports 160 MHz channels on Wifi 6E for a theoretical maximum 2.4 Gbps! Performance is screaming. Connected to my router at 160 Mhz. No need for the same SSID for 6 Ghz and all other channels. Maxing out my 1 Gbps Fiber connection from behind a concrete wall. This thing is fast!