Currently we only provide a low level plain ANSI C based SDK, compiled for relevant iOS architectures. We will soon (Q3 2020) provide higher level wrappers for simpler development.
This means for now you must use the Plain C Clients API. Fortunately, using a plain C-based SDK on iOS is relatively straight forward, especially if your project is Objective C based.
If you use Swift, you must setup an Objective C bridging header - there are many guides available online specifically to do this when using a C Library - for instance this one. The raw C-types are a bit of a mess to handle from Swift, though, so introducing a simple wrapper in Objective C is recommended (and will as mentioned also be provided by Nabto soon).