It's not about the device you are using, the primary user is determined by the username you are using. I can help look into this if you need me to. I will need to verify the username you are attempting to make the primary user. In order to do so can you please verify your first and last name, full street address, and the phone number or account number associated with your services in a private message?
To send a private message click on my name "ComcastChe", then click private message me.