Let's put the number of devices to bed first.
The package you are on is basically the size of the pipe to the internet and the pipe is used to feed all your devices.
So, the pipe is a shared resource, if you have 10 devices connected, each device should get 10Mbps each on a 100Mbps package.
You shouldn't be experiencing disconnections, just slow speeds.
The Hub's wireless radios (2.4 GHz and 5 GHz ) are also a shared resource as only one device can talk to a radio at a time.
The more devices connected to a radio, the slower that network will become as each device will have to queue to talk to the Hub.
If a device has to queue too long to talk to the Hub, it will drop the connection.
It is a bit more complicated when you consider that a client cannot talk to the Hub if a neighbouring wireless network is broadcasting on the same channel at the same time.A client will be sharing airtime with not just clients on your network, but also with all devices on networks in range, broadcasting on the same wireless channel.
There are a couple of things you can do to offset this, like balancing the number of devices between the 2.4GHz and 5GHz network, setting the Hub to use less congested (non overlapping) channels, or additional Wireless Access Points. You will need to separate the 2.4 GHz and 5 GHz bands as advised above to load split.
Being late to the party I have reread your OP, and the connected but no internet message could suggest a local circuit drop, but without collaboration with a wired device, hard to say. (there are many other reasons)
Starting a Broadband Quality Monitor may shed some light. However, will be no substitute for VM checking your connection and FECs.
I would split the SSIDs as advised (if not already done so) and turn off channel optimisation.
Maybe you could also provide a bit more detail on issues you are having, exactly what issues you are experiencing, on which devices, the band they are connected to and what error messages you are getting.
I am afraid wireless and broadband can be complex so as much info as possible can help.