Trying to do this by memory... At some point during this process I had to call Volvo customer service and have them activate my service because my dealer didn't do it. They wanted to know my PIN number.. I didn't know it so they reset it and gave it to me over the phone and adamantly said don't lose it.. So, once the Volvo On Call was activated thru Volvo, I did this:
From the home page, swipe left. Select the Volvo ID app and make sure you're signed in to the Volvo ID you created using the Volvo On Call app on your smartphone.
Then make sure your internet thru the cell modem is turned on by swiping down on the little tab at the very top center of the screen, going to settings, choosing Communications, and then I think it's Vehicle Modem Internet. Make sure that checkmark is turned on.
From the same "swipe left" menu, there's a button for Download Center. You use that to search for new apps such as Spotify and Pandora.
The internet cell service you get works on AT&T's 2G network. It is painfully slow. The apps are slow and songs buffer when starting. The apps also aren't very well developed so just not as polished as the smart phone versions. The Spotify app is handy when I don't want to deal with plugging my phone in to access Apple Car Play; but doing so usually means a better experience, in my opinion. I haven't even activated my in car hotspot for the same reason. Tethering to an LTE connection thru a smartphone is a much better option when available.