It is possible to enforce global proxy on iOS devices only if that device is a supervised device. This is not a limitation of Barracuda or any other MDM solution but rather a design decision by Apple it seems. In order to make a device supervised it can be done by Apple at the time of purchase by using the DEP program they provide. Existing devices can be configured first using the Apple Configurator to make them supervised.
Below is a link to a video series available via iTunes that describes the process of using Apple configurator.
Additionally, In certain circumstances it is possible for existing devices to enroll in DEP. Apple may be able to retroactively associate your devices with a newly created DEP account. Once the devices are associated with the DEP the customer would then be able to use our MDM service to supervise the device. DEP can be quite complex so you should confirm with Apple about your particular situation. Apple provides DEP information here https://www.apple.com/education/it/dep/
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