How to add xFI Complete without changing internet speed?

I work from home and spend a lot of time on my computer, so I've been going over the 1.2TB data. I would like to have unlimited data by adding xFi Complete.

The issue is, the xFi Complete is under the EQUIPMENT AND ADD-ONS section. When I try to get it, I can't access that section without changing my internet speed to a lower or higher one or adding another service, because it doesn't let me access the next page without changing something about my services.

I just want to keep my original plan (Just 300 Mbps internet), but add xFi Complete to it. I like the speed I have, all I need right now is the internet, I just want to pay an extra consistent fee for unlimited instead of being charged for going over.

Is there an option I'm missing? Do I have to choose a different internet speed or add another service in order to get xFi Complete?

If I choose a lower speed just to get xFi Complete and then upgrade to the speed I want later, would I have to do that?