On my network (Orange F) this seems to be an actual feature of the modem of the X10iii when my SIM is in slot 1. The assumption is that the OS supports IPv4 to 6 bridging on the device, a mechanism known as CLAT. We’ve been experimenting with this, and here’s a test implementation of CLAT in connman. We’d appreciate further testing from people who don’t mind getting their hands dirty.
In 4.5.0, a timeout in connman was increased slightly for networks that are slow to supply an IPv4, but really that’s still a fallback for IPv6-only mode, so that shouldn’t be needed when CLAT works.