When deploying Nintex App Studio apps to iOS there are really good walkthroughs in the following posts.

Generating a distribution certificate‌

Generating a provisioning profile ‌

Introducing Nintex App Studio‌

But I use a regular developer account with Apple when testing an App before deploying it using the Apple Developer Enterprise Program 

There are very minor changes to the steps to follow when deploying the app using a non Enterprise account. The simple benefit is that I am just using an individual account, and can only deploy the app to a select few devices before the app goes out to the rest of the user base using the Enterprise profile. NOTE: you must use an enterprise profile to deploy for production purposes.

In using the steps outlined in the linked articles, you get the certificate the same way but in the provisioning steps you get to step 10 and instead of selecting In House under Distribution you select Ad Hoc.

Then continue the steps 11-14 (14. On the Select certificates page, select the certificate you want to include with your provisioning profile.) and before you finish you are asked to select a device.

Select all the devices you want to deploy the app to and select Continue then it will be generated. Then download and use this provisioning profile.

Adding Devices

So in order to get this to work you have to first add the devices to the account to test with. To do this,

1. Under Devices select All. 
2. On the upper-right, click Add ( + ).
3. Provide a name for the device and paste the UDID of the device
   1. To get the UDID of your device: 
      1. Launch iTunes and connect your iPhone.
      2. In the right pane, locate the information about your iPhone, including its name, capacity, software version, serial number, and phone number.
      3. Reveal the Identifier by clicking on Serial Number:.
      4. Copy the Identifier to your clipboard by choosing Edit → Copy.

After finishing the steps as normal by uploading the provisioning profile into App Studio, I deployed the app, got the email of the finished deployment, and downloaded the app as normal.