ZTE has officially confirmed that they have started rolling out Android 7.1.1 nougat update for the axon 7 mini.
The update brings all the usual Android nougat features, but is also bringing an exclusive feature to the axon 7 mini that is not even available in the axon 7 which is the ability of calling through wifi only if the phone is using a T-Mobile SIM card.
ZTE has mentioned that the axon 7 mini should be least 30 percent charged in order to download the nougat update. The update is an OTA (over-the-air) update so you can go to the settings > System update to download the update.
Since it is an OTA update it might take some time to come on every device.