Extended the range and effectiveness of a router

You have two major choices in types of products: Powerlines (PLA series) or Extender/Access point.

The very best option for a stable connection and good signal is to drag/pull a cable from your router to your second floor. Then connect it to an access point to send out signal.

Alternative if you do not want to or can not drag a cable is to use the access point as repeater/extender which means you repeat the wireless signal. The negative part with this is that you get around 50% of the speed on the repeated signal. A repeater you would position where you still have good signal but far enough from the router that the extended signal actually covers the area you currently have poor signal at.

Access point/repeater products:
High performance: Armour X1 (Also known as WAP6806)
Medium Performance: WRE6505 v2

Then you have the powerline products which sends the signal through your powerlines instead of over wireless. So you can connect one ZyXEL PLA unit to your router and the other where you need better coverage/signal. Here you could connect a PLA5206 v2 to your router and a PLA5236 where you need wireless signal. The PLA5236 receives the signal from the PLA5206 v2 over the powerlines and then sends out a wireless signal that you can connect the units to.

Hope this helps!