Thank you for your explanation and the other comments.
BUT: If I insert the SIM card in my cellphone I have an IPv4 and IPv6 in parallel according to the IPv4/IPv6 check in the internet.
I would be very happy if you could advice more detailed how I can manage IPv4 and Pv6 in parallel on the RUT240 according your proposel:
"To get both IP you should create another mobile interface and separate PDP types. It would be easier to configure if both interfaces have different APN and PDP type."
Unfortunately I'm a newbie with RUT240.
Remark: I have a second RUT240, might this be dependant on the hardware? Should I try the second one?
Thank you very much!