new modem (ASUS) self install, contact by phone, online support

Setting up a new modem next Tuesday and I am wondering how does self install work? Fees?? I tried with my current modem just for testing and it shows,

"Oops! You do not have access to the Self-Install experience because either you do not have an open order that requires installation or your device is not eligible for Self Install at this time. If you believe you've received this message in error, please contact us."

Is this error only appearing because my current modem is already installed? Will this error not appear when i get my new modem (certified by cox)?

If i were to call by phone what will they ask? How will they help me by setting up my new modem?

Will it be the same if i were to go online support other than on phone?