Xfinity gateway wifi not reaching devices

Recently moved to a new apartment with Xfinity gateway wifi equipment pre installed. I activated the equipment and I have some issues. First, there are a few devices on the network I don’t own. The app lets me pause them but not remove them. Second. My Amazon echos and tv are not receiving consistent signal. It take several attempts to get on Netflix and Hulu. And my Amazon echos are down most of the day. Anytime I give a command to play music or do something they say they cannot reach the internet. I’ve run all the diagnostics but the app says I’m receiving 120% of my speed. I restarted the modem. I even took it into my local store to swap it out for a different one but still have the same issues. The wifi on my phone and laptop is a lot more consistent and works when the echo and tv do not, so I don’t know what the issue is. I tried restarting the echos and resetting the TV but again no change. Anyone have the same issue and find a viable solution? The e customer service reps just keep telling me my internet connection is fine but I didn't have these issues with my other provider.