A useful IPTV subscription is not always the cheapest one. It should be stable, compatible with your device and honest about what is included.
What to check first
Before committing, check whether a trial or short plan is available, what support is offered and how renewals are handled.
Practical Firestick advice
The best test is practical: load it on your actual Firestick, refresh the guide and watch the categories you care about at the times you normally watch TV.
Use IPTV apps and playlists only with sources you are authorised to access. A player app does not provide channels by itself.