Share a drive from a modem to a router

I have an external hard drive connected to a server which is connected to my modem. That modem has a router connected to it, that router is responsible for my wireless network at home. All of my computers at home are connected to that router.

I would like to share that external hard drive connected to my server across my wireless network, but I can not. When I try to add a network location or map a network drive it fails to find it.

I have shared the drive from my server but I am stuck past that.

Can anybody point me in the right direction?

Thanks!