Wow, some bad info in here.
1. You don't need to rent Comcast's modem if you have their landline service, completely false. You will have to get their modem to keep the landline service but you shouldn't need to pay for it. You can have a separate modem for internet and you should.
2. Internet speeds aren't the same as network speeds. You're streaming content from the internet to your laptop, then streaming it back through the network through wifi to the apple tv.
3. Wifi extenders/repeaters are mostly garbage. Save for maybe a simple smart bulb or switch that just needs internet connectivity.
4. You should never have to disconnect devices from your network or reboot your devices unless you're doing firmware updates. Once a week? If your ISP is telling you that, get rid of them and/or their hardware.
If I was to guess with the limited amount of info here, i'd say you have 2 issues: comcast's hardware (i'm going to guess yours is a little older) and interference/signal degradation. You mentioned hardware being upstairs and you're downstairs and i'd guess your house is older...all issues for hardware that is already struggling. Too much interference that prevents a consistent stream. Of course, all speculation based on the limited info.
For reference, I have 200mbps internet from comcast and have had my own modem for 5 years now. I redid my network 3 years ago with Ubiquiti's Unifi system because I also had hardware that couldn't keep up. I am now on my work computer on calls/video constantly while my wife is on video calls as well and the kids doing their school stuff on ipads while streaming music and download/uploading assets to our web servers and not a hiccup at all. It helps that the hardware i'm using is prosumer grade and not consumer grade. Consumer grade equipment is just horrendous these days.
