If you are looking for a way to avoid buying expensive apps from the Amazon store or not wasting money on expensive subscriptions, but continue to use this brand's streaming device, then you need to install VPN on FireStick and use third-party apps. But you should be careful about this. To find good and legal free streaming applications, I advise you to look for lists of such sites and developers on trust portals, for example, in this post
https://www.firesticktricks.com/amazon-fire-stick-apps.html is a description of many of them. This will help you save your money and not get problems with the OS of your device.