On Android, you can just go to "Settings" -> "Cellular networks" -> "Access point names" and choose whether you want to request an IPv4 or IPv6 address. As far

On Android, you can just go to "Settings" -> "Cellular networks" -> "Access point names" and choose whether you want to request an IPv4 or IPv6 address. As far as I can tell (from looking at https://support.apple.com/en-us/HT201699 https://support.apple.com/en-us/HT201699), iOS doesn't have an equivalent option, so it probably just doesn't support IPv6 over cellular networks yet.