Thanks @Werner for the quick answer and also @hed.
As @hed says, I already have a public ipv4 address on IB3, which is unfortunately static/almost never changed. As @hed mentioned, for security reasons I would like to prefer a dynamic one.
After rebooting IB3 it always gets the same ipv4 address and apparently it wasn’t changed automatically by default.
You might have to call Swisscom support because I can’t find a function/setting option in the IB3.
Thanks