@sleightflight 

"... so I connected directly to the router via WiFi and it was at 35mbps"

Hi,

Doesn't that rule out the RE505X as the cause for the drop of speed to 35 Mbps?

What type of router are you using? (e.g. DSL, cable, fiber)

Is there a way to test your Internet speed directly on the router?

  @woozle 

Hi, basically proves that the drop is the AX1500, as the low speed is only if a device is connected to the AX1500, be it pc, ipad, phone , be it as a WiFi range extender or by plugging an ethernet cable into the AX1500 all show low speed when connected to the AX1500, but connecting to the sky router the speeds are over 400mbps.

  @sleightflight 

Ok, so the broadband connection itself is fine then.

If you position another WiFi-capable device at the location where the RE505X is, connect it to the 5 GHz WiFi of the Sky router and run a speedtest, then what kind of results can you get? Substantially more than 35 Mbps? If yes, then check in the Wireless settings of your RE505X whether it is connected to two "Host Networks" (2.4 GHz and 5 GHz). In case it is, then disable (i.e. uncheck) the 2.4 GHz Host Network connection to force the RE505X to use the 5 GHz band for connecting to the Sky router.
 

  @woozle 

Hi Woozle, 

Thanks for the reply, I do have both bands switched on as I have some devices that can't use the 5ghz band. 

I've had this device now for over a year and the speed has always been higher when being upstairs and further away from the router connecting to the AX1500, it's only a few days ago and for no reason it's suddenly dropped. 

I will try disabling the 2.4ghz channel, but I have a feeling it won't cure the problem. 

  Hi, OK I switched off the 2.4ghz, speed with the AX1500 with it not far from the router was OK, taking it upstairs it could no longer connect to the 5ghz Internet, AX1500 reporting no Internet access, so something has gone wrong with the AX1500 that it can only connect to the router 5ghz when it is close to it. I have a new upgraded adaptor that has come today and the speed on the new one upstairs is absolutely fine. 

  @sleightflight 

OK, that definitely sounds like some part within the RE505X that is responsible for the 5 GHz band has got a problem. And since the RE505X couldn't connect to the router via 5 GHz anymore, it switched to using the inherently slow 2.4 GHz band.

By the way, the extender's "Host Network" connection settings don't affect your client devices. They are solely for the connection between the RE505X and the router. 
The bands available for connecting your 2.4 GHz and 5 GHz client devices are determined via the settings in the "Extended Network" menu.
 

  @woozle 

It is still or should be still under warranty, so I will raise a ticket. 

Thanks.