Your post suggested that your firestick was previously working fine at expected wifi speeds? Were there recent changes?
- VPN recently installed or used on firestick?
- Recent firmware or software update on firestick?
- Firestick getting hot?
The firestick has had issues with heat if location does not provide adequate cooling - prolonged high thermals have caused variations in hardware failure, possibly affecting its wifi adapter.
Things to try (if you haven’t already).
- Reboot firestick and retest speeds.
- Reboot router and modem.
- Change wifi bands as @Miki suggested.
- If using vpn, shut off and retest speeds.
- Check speeds after device is off for a while and cool.
- If wifi adapter is suspect, consider getting ethernet adapter (link below).
Amazon Ethernet Adapter for Amazon Fire TV Devices https://a.co/d/cUSSxu9
-
If determined as device failure - consider getting a new device or see if still under warranty.
* Look into a low cost Onn 4K box as alternative. -
Also note - 4mbps is good enough bandwidth for 4K streaming, unless there’s random dropout in speeds.
Good luck.