Keep us updated on what you figure out.
My Pixel 2 XL's would not roam no matter what I did a couple months ago in my market in areas that roam beautifully (on US Cellular).
Both devices were on 9.0 Pie (August release). I haven't been back out there to test since...
I don't have an S8, nor do I know anyone that does. However, I thought that T-Mob roaming would need to be active in the market for Sprint to launch VOLTE in it? I could be dead wrong...