On Android, I can set my APN to be either IPv4, IPv6, or IPv4/IPv6.
I'd imagine the same option is available on iPhones. I just don't know where.
I did some further research, and it's an Apple limitation.
They apparently won't let you connect to anything besides IPv6.
So, you'll have to take this up with Apple.
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I also have a Verizon iPhone and it connects to VPN just fine over Verizon. In fact, when I connect my iPad to my iPhone via hotspot, the iPad can also VPN. As soon as the iPad is back on T-Mobile LTE, it can no longer VPN. It's a myth that this is an Apple limitation.