Apple has added an option to enable ipv4 optional, described here:
I don't know openwrt's state on various ipv4 tunneling methods?
CLAT works for me, at least on Orange Poland mobile network, but there is one OpenWrt-specific quirk. If the 464xlat package is installed, then the sub-interface is added unconditionally to every dhcpv6 interface, even dynamic one, unless explicitly disabled by option iface_464xlat '0'.
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