abasel
Is it better to configure the router as a modem or should I leave the modem separate?
Typically you would configure your modem/router in bridged mode, which just passes the signals through to your SME server which is configured for gateway server mode.
Your server then handles the login client, DHCP server, DNS functions, firewall etc. There is no need to do network maintenance or adjustments in your modem/router then as your SME server handles everything including the firewall.
If you need or have specialized requirements for firewalling etc, then your router does all the functionality mentioned above (configured in normal modem/router mode), and your SME server is then configured for server only mode.
If you need to use the wireless function that many routers have, then you need to use your modem/router as the gateway/firewall etc, and only use your SME server in server only mode.
It will be easier to administer a bridged modem and a SME in gateway server mode, as all/most configuration is done via server manager.
If you use a seperate router/firewall then you have to setup port forwards for all services you want to access on your SME server, eg web, https, mail, ssh and whatever else.
You obviously must disable all these functions in the modem/router.