@nashienet 

You state you are using the C5400 as an AP but are you using it in AP mode?  

yes i am

@nashienet 

Because when the router is put into AP mode all of its routing functions are disabled.  Some of the problems with the connection speed may be comming from the communication between the C5400 and the router its connected to.   A few things to try to determine the issue

1.  Make sure the "router" doesn't have wireless enabled as well.  if it does disable that wireless so that only the C5400 is broadcasting

2.  Connect a PC dirrectly to the C5400 via ethernet and test the connection. 

3.  Make sure the cable between the "router" and C5400 is a Cat5e or Cat6

4.  If possible put the C5400 in as a router and test the connection

Best Regards 

Hi,

The main router is a draytek vigor 2925 and does not have a wireless ability at all.

When I connected a wired connection to the c5400 it reports a 1gbit/s connection and a broadband speed test gives me exactly what I would expect.

The ethernet cables in the walls are cat7, the cables from the ports to the router are cat 6. But as the wired connections work as expected it shouldn't be a cabling issue.

I will put it in router mode and just disable dhcp and use my main router as the dns and see how that performs (that's how the legacy tplink router worked).

I did reach out to tplink support for help, after an initial response I haven't heard back which is a shame as I replied with screenshots and the feedback requested (sent that on Sunday). I forwarded it over again on the offchance the email didn't arrive but still nothing unfortunately.

Thankyou for your help, it's much appreciated. 

@nashienet 

Let me know if putting the C5400 in router mode solved the issue you were experiancing.