Difference between modes?

Someone suggested switching from router mode to extender mode web AP for better performance.  What’s the difference between these 2 modes, and what are the disadvantages versis the advantages?  Trying to keep my MBL duo online, but doesn’t seem to be coroperating with me.

My fault, using the MyNetN900

In extender mode the device will behave like a switch, it will not do any routing.  The “Internet” port becomes switched with the LAN ports.

If you already have a router you can plug this in using extender mode and have all the devices on the LAN side of your router available on your local subnet. 

 All of the options associated with QoS priority, port forwarding, DHCP, PPPoE etc rely on routing so they won’t appear in extender mode.

If you don’t have another router then you probably need to use router mode.