[MovedGoalPosts]

Are you SURE this is actually a router and not just an ADSL modem with 4 network ports on the LAN side?

I suspect you need to get a proper router. Connect the WAN side fo the router to one of the ADSL modem's ports and assign one of your fixed IPs to your router's WAN setup. You then connect your LAN side of the router to your networked PCs allowing them to obtain their IPs from the router's DHCP.

Of course how well this will work depends on other aspects of your setup. As far as the internet is concerned it would see all PCs as having the same IP. That could create problems for some specialist software that couldn't work out how to get stuff onto a specifc PC.

To be honest, if you are running an internet cafe, this is part of your livelihood. I'd be seeking advice and support from a computer consultant to ensure that it all stays up and running. Are there limits to how many PCs your ISP expects to be connected at once and would you be breaking your contract exceeding that? What happens if the ADSL dies - do you have a backup. Recently I found ADSL has no guarantee of connectivity, despite what your ISPs website might say.