You've done a great job troubleshooting, and I'm so sorry you're still having this issue!
The next step we would recommend is a factory reset of your Fire Tablet.
Before you begin, be sure to back up your device. To learn more, go to Back Up and Restore Your Fire Tablet. Please note, backing up doesn't save your account passwords or personal content. If you choose not to back up your device, your Amazon-purchased content is always available in the Cloud.
Individual app settings, game progress, or in-app items may be lost. Check with the app developer before resetting your Fire tablet.
To reset your Fire tablet:
Swipe down from the top of the screen and tap the Settings (gear) icon.
Tap Device Options, and then tap Reset to Factory Defaults.