Here is my (very well educated) guess about why the SIM you bought at a department store didn't work.

There are two broad categories of SIM cards in the world.

First are SIM cards that are branded and sold by an organization that actually operates a cell phone network. Think of Swisscom in Switzerland, Orange in France, EE in the UK, AT&T and T-Mobile in the USA, and Rogers & Bell in Canada. A SIM card sold by a network operator will always work as long as you are nearby a cell tower belonging to that operator, something that you can take for granted if you are buying the SIM card from the network operator's retail store.

The other category of SIM cards are those sold by Mobile Virtual Network Operators. These companies don't possess any network infrastructure (cell towers, switching equipment, etc.). They work by negotiating agreements with actual network operators to buy connectivity in bulk from those operators who actually own & operate a network. But, the big problem with MVNOs is that they might only have an agreement with one actual network operator, and if you are not in range of a tower of that operator, you are out of luck. Furthermore, the contract that the MVNO has with their actual network operator(s) might limit their access to only 3G service on a network that is actually 4G or 5G, or it might throttle data speed.

You can generally identify a MVNO by the branding on the SIM card. If the SIM card is branded "7-11" (a large convenience store operator in the USA), or branded by an organization that you have never heard of as a cell phone network, chances are it is a MVNO SIM card.

For travellers, it's generally best if we stay away from MVNOs and only use SIMs from actual network operators. This is because we are moving around all the time.

MVNOs are great for people who stay in one town all the time and thus know that there are cell towers in their town operated by a partner of their MVNO. They are also great for light users who don't need high speed data, something that we travellers generally want and need.

Hope this sheds a bit of light on things.

Michael