In this context I have been PM’ed about how to find out whether the connection is via IPv4 or IPv6.
As this might be interesting for others, too, here is the answer:
There are various web-services, for example
http://get.youripfast.com
showing your public IP as well as the IP version in use (IPv4 or IPv6) for your device.
Note that all devices that are behind a router will show the external IP of the router.
So regarding the issue discussed in this thread, you could deactivate WLAN for your smartphone (has the external IPv6 connection of the router) and establish a mobile data connection with it (2G, 3G, 4G). Your mobile carrier will probably assign an IPv4 address to your phone then and you can register with MARTI using your smartphone, then.