Depending on your needs you might find that you're just as well off using an old phone or buying a cheap new phone and setting that up to use as a WiFi hotspot.

You might also consider changing your mobile data plan to something that is higher or unlimited and just using your current phone for this purpose - if battery life is a concern then a backup battery would be cheaper and multi-purpose as it would be useful for your phone itself as well as extending the amount of time you're able to run the hotspot for.

The device above is useful for particular use-cases but tbh if you're looking for something separate to your current phone then a secondary phone may be a better use of your money (or free if you have an old phone lying around), or investing in a backup battery means you'd have something about the same size as that device but you'd be able to get a lot more mileage out of your phone while costing a lot less.