Don’t know where you are located so the rules vary. I’m in the US and use prepaid Speed Talk Sim cards that run on the T-mobile network. They start at $5 a month, no contract but offer auto fill with a CC. They are available on Amazon. Your carrier may offer a data or phone card but it won’t necessarily be connected to your account. It is usually freestanding just like Speed Talk cards but usually more expensive. Carriers want you to buy on of the watches they sell and none I checked with will allow you to add your Android watch to your account.