@Nathan123 

There's a couple of other people experiencing the same issues, see the US forum:

https://community.tp-link.com/us/home/forum/topic/202492

Hope we can get this fixed ASAP

@Nathan123 

Hi All, 

thank you for your responses, I went dark over the Easter break to spend time with my family (I'm usually always working).

I've actually solved this, though by accident, basically by clicking things. I know..I know... In any case, I wanted to share in case anyone else is getting the same issue.

It seems that there was some type of bandwidth output limitation in the deco unit. It was receiving the full 90mbps speeds from the modem (shown when I used the app to test the speed) but nothing connected to the Deco wifi recieved 40mbps. I have never set this, I couldn't even if i wanted to. All I can think is that it tested the speed from my modem when I installed it (I was on a lower speed plan), and it set the max bandwidth at that speed automatically. Just a guess.

I went exploring through the Deco mobile app and started trying to prioritise devices so see if that helped (prioritising my laptop which I use for work) and for some reason, when I clicked to priorotise my phone (and it only happened with my phone) it asked if I wanted to set my maximum bandwidth manually or automatically through a speedtest. I clicked auto, it did the normal speedtest I'd done before, and that I got the full speed across all devices to the correct 90mbps.

Since that, I've not actually been able to find where you set the maximum bandwidth output, so I couldn't replicate it. I'd need to rely on this to randomly pop-up. Maybe I missed it though. I have a 3 year old and 1 year old daughters who scream and hang off me constantly, so it's hard to focus.

The story here though is that it you are getting the same issue I am, try and reset the maximum bandwidth. I can't tell you how but maybe other users can? 

Or if you are reading this and know how to do it properly, it would be great to share in case others find it in the future and don't have to rely on my poor solution!

Thank you again for your responses and good intentions to help.

I hope everyone is staying safe in the current COVID-19 pandemic.