Now I understand the question(s)

( Will reply in english in such a way that other peoples can follow issue. If you have problem, I can translate it later ).
You are using a modem/nat router to connect your sme to the internet, and SME runs in ' server-only ' mode.
It should be OK ...
Would suggest to check the following :
1) Did you put the 'intranet' on the right NIC (suppose there are two nics in the SME, since it was Gateway)
2) Are you sure all IP's are compatible, that you do not have conflits from IP's coming from the router (dhcp from speedtouch ? ). Server must have fixed ip within your intranet range.
3)Portforward on your router (speedtouch) should work.
I guess you are only forwarding port80 to your SME.
4) Take a look to your cabling/hubs/switches/Nic's. Cabling or bad hub/switch port or bad NIC is from time to time the cause of such problems. Also try without the 'speedtouch' connected to the network ( for example ).
Hope this can help.
- Hervé