Extender WN3000RP connection to an Optus NBN Router

I have recently connected to the NBN (This is Sydney and it's for Fibre Optic connection). My computer speed with direct connection to modem is just below 100mbps and on wifi about 87 Mbps.

Prior to this I was connected through cable with about 30MBPS. My Extender WN3000RP then gave me, from two rooms away, speeds of up to 20Mbps on my iPhone and iPad.

Now that I have connected to the NBN wifi speed is between 0.38 and 1.00 Mbps while susing the extender.  I have spent a couple of days searching and re connecting. I have also upated the software on the Extender WN3000RP and reset it through the browser.  Nothing seems to work.  Surely the speed should be faster than I enjoyed when on cable. Is it possible that the Extender WN3000RP is not compatable with the wifi through NBN?  I even phoned support yesterda but after 50minutes waiting for an answer I hung up. No one has all that time to waist waiting for some one to answer a call.

NOTe: to enable posting this email I am forced to select a Model from a drop down menu. I have no idea what this is as I can't match anything to anything on the list. I have selected the top one just to enable me posting this email. 

Would appreciate anyone's input.

CAS