Asus DSL-N55U settings worse than PN's Technicolor TG582n

[#1]

Having just switched from the PN issued router to a new DSL-N55U I am disappointed to see the download speed has dropped from about 5.6 Mbps to about 4.6 Mbps as the attached image shows.  Switching back caused the download speed to revert.  What settings need to be adjusted please ? 
Currently I have
Latest Firmware Version:3.0.0.4.374_4422
PPPoA  VPI:0  VCI:38 
Encapsulation Mode: VC-MUX 
MTU: 1492
Wired connection.

[#2]

Hi, those settings look fine, you might want to try the MTU at 1430 or 1458 to see if that makes any difference.
I assume the speeds you mentioned were achieved over ethernet rather than wireless?

[#3]

Changing MTU had no significant effect.  Am I right in thinking that I can change the MTU then run a speed test almost immediately to get valid results ?  In fact I think testing almost immediately would slightly flatter the results of larger packets as there is less opportunity for retransmit requests.
Yes, over Ethernet not wireless. 
So frustrating to have so much added control yet not be able to even match the default settings on the relatively cheap router.
I've tried Asus but they have not been able to help.

[#4]

Every router had different sync rate on ADSL/ADSL2+ depend on the router chipset inside. My Netgear DG834N only give me 14.4Mbps while my other router Netgear DGND3700v2 give me 16.5Mbps

[#5]

Have you tried tweaking the 'Stability Adjustment' setting in Admin\DSL Setting ?  That might nudge your sync speed up a bit, unless it's not syncing lower, it is just the dl speed?

[#6]

I done it before but I am no longer on ADSL2+. I am now on Fibre for five weeks now.

[#7]

Yes.  The default stability gave me a download speed of a little over 1 Mbps.  I changed it to -5 dB to get the speed up to today's value.  I have not tried any other values for this parameter, perhaps some experimentation would yield better results.  The following is an indication of line quality when the ISP's Technicolor TG582n is in action.