I'm getting a NE113 error trying to enable LTE on my Apple Watch. It seems like it's related to missing VoLTE provisioning. I've encountered a couple similar threads but it seems like there's nothing I can do on my end other than contacting the support.
For those who's going to stumble upon this thread in the future: NE113 error is indeed related to VoLTE, my initial guesses regarding inability to resolve the issue myself were correct. The support was quick and friendly enough to enable the necessary functionality on the provider side and now my watch has LTE.
I don't know why do I need LTE on a watch though, but we'll see.