I would think your problem is an overstuffed wifi channel.
In my case, I was having all sorts of WiFi issues while my router was set to select its channel automatically. What I did was run an android app to find a less travelled wifi channel. I changed to that channel and haven't looked back.
Then my daughter had a similar issue in her home. I did the same thing to her setup and she's also very happy with the result.
We both are using Firestick 4Ks also.