I have a T-Mobile SIM that, on a OnePlus One, works perfectly fine on T-Mobile's APN in IPv4-only mode, with an IPv4 address and everything.
However, when I take it out and put it on a OnePlus 5, it doesn't work with the same APN settings unless I change IPv4 to IPv6 (or IPv4/IPv6).
I'm baffled at how this is possible. How is this possible? Shouldn't all phones be able to connect to the same APNs with the same settings?