@NickEnglish 

If this is your new Sky router, check Sky support documentation where it says it is normal for Internet speed to fluctuate during first 14 days, as they "calibrate" your Internet connection. Have seen it written, got amused by that, but didn't bookmarked it, sorry. It is somewhere it troubleshooting documentation.

I run Deco mesh with M9 Plus in Access Point mode in my house, and quite pleased with Deco performance. The only time web pages got stuck is when my Internet connection goes down, which happens occasionally. Easy to verify, because LED on Deco goes red when Internet connection drops.

Other than that, Deco M9 Plus in Access Point mode is "power on and forget." 

It is (remotely) possible that perhaps you have not positioned Satellite Deco units properly and they have hard time communicating with each other. They need to have strong signal between each other, or you may have WiFi mesh performance issues.

To check that, in Deco app open each Satellite Deco one by one and on Satellite Deco page check what Signal Source shows. Example:

This shows excellent connectivity: there are three channels listed, one 2.4GHz and two 5GHz, and signal is three blue bars. Check every Satellite Deco, if any has less than 3 channels, or less than 2 blue bars, or yellow/red bars instead of blue - this Satellite Deco is in wrong location. Anything connected to it will suffer degraded Internet connectivity.

Also, you can check if WiFi connection between your mobile device and Deco unit is good. This is how: How to check the associated/link speed on a wireless client

If everything looks all right, Sky Tech Support tells you they are not "calibrating" your Internet connection, then you can also run the following test: power off all Satellite Deco. Leave just Main Deco running. Come to it with your WiFi device and run speed test on that device, also try web pages. 

With that configuration you have single Deco unit, meaning mesh functionality is disabled (not available). If you have issues with Internet even in that configuration, they are either between Main Deco and Sky router (ethernet cable), in Sky router or between Sky router and Internet. Not with Deco mesh as it is out of the picture.

@Alexandre. 
 

many thanks for the very full answer and I will test this tomorrow. When I first set it up I disabled the sky wi fi but did not put the Deco in Access Point mode. It seemed ok at first and then got very slow. Putting it in Access Point mode helped eliminate the, I guess, conflict between the two routers. However, what I seem to be finding now is a fast speed test but some web sites simply not loading at all....not able to contact the server. Some are fine.
 

This made me think it was perhaps something not to do with my wi fi configuration. 

BTW the Devo speed test does not work, says I am off line..presumably because it is no longer acting as a router?

thanks again

nick
 

@NickEnglish 

You should be able to run speedtest in AP mode. Just checked it in my Deco app, for Main Deco M9 Plus I have, it runs just fine, as usual.

Speedtest runs from Main Deco, if it fails all the time every time something must be wrong between Main Deco and Internet. Found this document for you: Troubleshooting: Slow Internet Speed with Deco Devices

Ignore Section 3 of that document, it is mostly for Deco in Router mode, not applicable to AP.

If you end up escalating issue to TP-Link Support by email or live chat, make sure you mention that Main Deco seem to be online, is connected to Sky router by Ethernet cable, but Deco app speedtest fails to run with "offline" error message.

Also, because I am sure TP-Link Support will ask you that, check that Ethernet cable is properly connected to both Main Deco and Sky router. Better yet, find Ethernet cable that came in box with Deco set and use it to connect Main Deco to Sky router.