Hi ,
If you are trying to install the license on the NCP client itself, you just need to be connected to the internet and then you can follow the steps provided on the below document to activate the license you have;
https://www.ncp-e.com/fileadmin/pdf/service_support/Entry-Client-Mac-Activation-e.pdf
The above pdf is for MAC OS, if you need for Windows or Mobile you can find the document at below link;
https://www.ncp-e.com/en/resources/software-activation/
Ypu can find the FAQs for the same at below link;
https://www.ncp-e.com/en/resources/faqs/software-activation-secure-entry-client/