LTE requires use of a voice SIM.This leaves two options, ruling out most other MVNOs because they only offer data:
available from b-mobile. This also means that visitors without residency cannot get an LTE signal with an iPhone. Also, this carrier profile does not allow tethering.
I have absolutely no idea why is the problem with LTE and data SIMs, or what is preventing the phone from being able to tether, even with a different carrier profile.