Question Network with modem -> switch configuration

It can't work that way....well it can if you pay the ISP lots of money but a new router will be cheaper.

The main problem you have is the ISP will only give you 1 IP address. This mean you can only connect 1 device behind the modem. Although you can technically put a switch behind a modem only 1 device will get that IP address and the other will not function. The only option would be to pay the ISP for more IP addresses and many do not even offer that service.

The main purpose of a router in a home environment is to share that single IP address between mulitple machines.

You really have no option the connection must go modem---router----switch/end devices.

To get above 100mbps you will have to buy a router with gigabit lan and wan ports.