Hi Moidcher,
Not being familiar with the web interface of this router:
You'll have to find where to set a subnet reservation, out of the DCHP pool range. Leaving plenty for DCHP to start with. If the default is say 192.168.1.1, the assign a pool to 192.168.1.100-254 ; this way you have ~98 static IP addresses, and ~156 for the DHCP, you could set it where you wish.
Though, it appears as if this router may handle the specific reservations setting itself. As pictured below.
For the second router, you could turn off DHCP, assign the second router a static IP within the range of of lose DHCP addresses on the first router (that is if you intend on accessing the second router itself for any reason after initial configuration)
Otherwise, on the second router the more simply way would be to leave DHCP on the WAN set, and turn off the internal DHCP itself, and allow the first router to manage everything.
On the second router, plug the cat5 into theWAN LAN, and connect this to a LAN port on the first router.
Here is the manual itself:
http://www.dlink.com/-/media/Consumer_Products/DIR/DIR%20850L/Manual/DIR850LA1manual041913v10US.pdf