S series switches require a DHCP acquired IP address for management and connectivity to Datto.
When an S series switch is plugged directly into a modem/upstream provider, the switch receives a Public IP address. It will check-in to Datto and report online. However, the S switches are Layer-2 switches, which means they do not have NAT/DHCP capabilities. This means that any device behind the switch will not receive a lease. If your upstream router is in "Bridge Mode" you may notice the same behavior.
When the switch is properly installed, it will receive a private management IP from your network's router and will pass DHCP information to downstream devices. Always make sure to check the IP the switch is receiving (under Manage > Switches) to be sure the switch is in the expected subnet.
Note: to ensure the switch always has the same IP address, add a DHCP reservation to the DHCP scope.