I believe that you have link to the ISP using 2 address pool (103.x.x.x) and it s public IP address range one at your router interface and other at ISP interface. Other range 112.x.x.x is what is IP address that you bought ... and ISP set route for this subnet that redirect all traffic to your router.
Now you have to do static NAT to configure your router when it receive packet with destination IP 112.x.x.x to redirect to your server inside your private network (192.168.x.x)
steps for NAT (cisco router) are:
Define static nat
Router(config)# ip nat inside source static 192.168.x.x 112.x.x.x
Define nat inside interface (probably gigabitethernet 0/0 or similar)
Router(config-if)# ip nat inside
Define nat outside interface (interface with link to ISP)
Router(config-if)# ip nat outside
And that s it... or you could use port redirection ... using your public IP from interface faceing ISP in that case define interface (inside / outside like in previuose step) and this
Router(config)#ip nat inside source static (tcp or udp) 192.168.x.x (port ... if you redirect to web server then 80) 103.x.x.x 80
still you need to register domain and connect domain with appropriate IP address depending of scenario you choose.