Start with the result from the TV, rather than a speed test from your phone. A useful reply includes the Fire TV or TV model, the network name it joined, the location of the router or mesh node and the times of two short tests. Say whether the adapter connects to the main router or to a mesh node with a wireless uplink. That changes which part of the route the cable actually replaces.
Use the same app and programme for both runs. Return to Wi-Fi after the wired test and see whether the interruption returns. If it does, try one ordinary change, such as moving the stick a little clear of the screen with the recommended extension or connecting to the nearest suitable access point. Record the result before moving anything else.
If the wired run also fails later, add the new date and programme to the same thread. A quiet ten-minute test still tells readers something if the later run includes a different time or load.