Change of mobile number reference

Hallo.
I am a new O2 DSL customer. The appoitment for the DSL connection is for the 21th of March 2019. During the contract signature procedure, I inserted my personal mobile telephone number as required reference. In the meantime I got all the information via email, and the confirmation of the appointment, but I am outside of Europe for work, and I will be back only after the appointment. I would like to change the reference telephone number with my girlfriend's, who is going to be there for me, but I don't know HOW to change the reference mobile number.

All the email communication says I could change the information in the web portal, but apparently I cannot sign-in as customer to the portal before the connection is done (paradox?).

Any help, tips, suggestions?

Thanks,
Stefano.