I've just acquired a second hand 4G/Wi-Fi Samsung Galaxy tablet. Most of the time, there will be Wi-Fi where I will be using it, but sometimes I'd like/need to use the 4G connection. In an ideal world, I'd get a no-ongoing-charge SIM card and buy "data packs" of various sizes as and when I needed them. Or, perhaps, a prepaid SIM with a data allowance that lasts for a long time.
Does anything like that exist?