I think there are some bugs on UE4.26.2 which is not support the old devices, i figure it out with the source engine,if you use launch version , i think you can try this:
use ARKitCameraMaterial from UE4.25 to replace M_PassthroughCamera from UE4.26.2 ARUtilities/Materials folder, ,change the name to “M_PassthroughCamera” then it should works, but be careful, just copy that material, don’t try to change the content inside, i don’t why, it always lack of a function(depreciated on 4.26) if try to change it and apply.
It works for all the AR function that i used, not sure if there is any effect on other feature like AR Occlusion.
By the way do you guys know how to implement AR Occlusion on IOS? I struggle a lot with that, please let me know if you have any hint