I was surprised that my dad's iPhone SE couldn't get 4G on his recent trip to Shanghai. He was using the same AT&T int'l roaming service (the $10/day add-on) that I used to get 4G on my Galaxy S7. (Although he was able to get 2G using the AT&T SIM.)
Furthermore, he went to a China Mobile shop and the attendant took her own China Mobile SIM and placed it into my dad's iPhone. That SIM also could only get 2G. According to the attendant, only iPhone 5S and earlier models can get 4G using China Mobile SIMs. Something happened with recent iPhone models.
Considering iPhone tech specs show they support the widest set of global bands, I'm surprised. Most likely, this is Apple trying to control user experiences again..