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Hi, I live in a 4-floor house (1 floor is the basement) 6 family members live here our internet is slow. Our modem is on the 1st floor and I also have a 3 piece Eero wifi pack. My current plan is up to 100mbps download, and 10 Mbps upload. After talking to a cox tech support they say that it supports 5-7 devices. I will list the devices that are connected down below if it matters.
Device rundown(I added everything that connects to the WiFi):
3 PS4 game consoles
1 Xbox One
2 Apple TVs
1 Smart TV
7 Computers(One streams music constantly because we have a radio station.)
3 apple watches
Slow Internet is an ambiguous description. Latency (slowness) could be your WAN connection to Cox, your router connections to each device or the size of your data plan. You'll need to measure each connection. I'll assume you have measured 100 Mbps with a wired connection to the only LAN port on your modem, so, if that matches your data plan, that's not the problem.
The number of devices is not the concern but what the devices are doing. If everyone is checking email, no problem. If everyone is streaming video, there would be a problem.
From the outset, I feel it's your data plan. I don't feel 100 Mbps is enough for 28+ devices. Even if something is online and not doing anything, it's still consuming bandwidth. Bump your plan up to 300 Mbps.
If you're only streaming as a management tool, I'd set your "quality" to the lowest audio setting.
I don't know much about Eero. Is it a good router?