Yes, you do not need a Danish mobile number for the 3-D Secure solution. When enrolling the mobile number, you need to enter the country code and the mobile number in two different columns. Do not use "+" or other symbols in front of the country code.
Note: Some countries block SMS from foreign countries, and SMS from Nets can therefore not be received by some foreign mobile numbers. In this case the recommendation is to use a Danish mobile number or MitID – both should work, even when you are abroad.