I have tested a device developed by the company I work for on a private LTE network. I was provided SIM cards, which were programmed for their particular network.

In Android devices everything worked as expected. However, none of the iOS devices associated with this network, although the devices did support the LTE band for that network.

Speculation was that there is a PLMN (cellular provider identifier) whitelist in Apple devices, which could not be worked around.