Question about BT Internet Complete WIFI

Hi,

I have a Smarthub 2 from BT Internet, but have crap WIFI reception upstairs.

I've seen BT do a black "complete wifi" disc that gives you mesh internet, but you have to pay more per month to get it. I'm not interested in the white discs by BT, as they effectively replace the router.

I'm tempted to buy a black disc outright, as you'll save the money over time by not paying the addition monthly.

Do you NEED the subscription for the black discs to work? Does BT have to activate the disc via the subscription, or is it plug and play?

I cant find an answer anywhere, so was hoping someone could help here.

Thanks.