It should work. Try updating iTunes.
If all else fails, install Windows (either via Boot Camp or VMWare Fusion 8.5.10 (last version for High Sierra and these CPUs), or if you want a free solution, then Virtualbox) and then you can get the latest iTunes version there.
Remember this is not only iTunes but the support payload, most specifically drivers and apple mobile device support tools that are needed, and on Windows these go hand in hand, on Mac OS it is usually up to date and gets updated with system but if you are on a legacy/nonsupported OS then after a few years, Apple leaves you behind in this regard. This may be happening for High Sierra, or might be happening in the coming years as new iOS devices are released.
But these machines are perfectly capable of running Windows, which, in turn, can have up to date itunes software. Ironical how you might have to install Windows to get an Apple device to communicate with another Apple device but this is what will definitely fix this. Intercompatibility and standardization at it best.
good luck