lional
If your sme server is in server & gateway mode, & your router is NOT in bridged mode (which I guess is the case) then you will probably have issues.
Change the server setting to server only (in Configure this server screens of console), & select the routers gateway IP as the Internet access gateway (again in Configure this server).
You would only be using one network card in the server in server only mode & this would connect directly to the LAN ethernet.
Depending what you are trying to do with the sme server you may have other issues
eg you will need to forward ports from the router/gateway/firewall to sme server if you want to run a mail server, & allow external access to IMAPS & SMTP server, & provide ssh access etc
As Stefano says, you need to provide us with much more information about your setup & what you are trying to do etc.