Sorry to bump this but Ive seen this issue referenced and acknowledged in a few places in the forums and over the internet, which I would describe as:

"Deco internet speed limited to 230-250mbps in router mode, but not AP mode"

I'm in the above boat. I have checked all the settings, removd QoS, checked direct from modem that I am getting 900mbps, and am using cat6 cables. I use my Deco m4s in router mode. Can I ask, why does this limit exist in router mode? I have my ISPs router and can probably, with some work, make the most of my 900/300mbps connection. But I have a decent amount of work set up in port forwarding, ddns etc. I don't want to lose the deco app functionality and ability to configure these things from my phone remotely.

I have no high priority devices and set my Down/Up to 1000/1000 in the qos.

Is it just not possible to get more than 250mbps in router mode, and why? Switching to AP mode isn't a "solution" in my opinion. It's barely even a "workaround" given it requires more hardware (router), external ddns, and an entirely new setup of your home network on your new router, which in my case doesnt have a nice app for configuring.

Update to this: have made the switch to using routers ISP and Deco in Access Point mode and can now get much faster speeds over wifi and ethernet, however I HATE my ISPs router and management console, so I only see this as a heavy trade off and short term test to see what headache I'd prefer: unutilised internet bandwidth or complex cumbersome network setup with poor config options

  @Khanage 

I'm also seeing this.  Had fttp installed yesterday and the isp insist their router is used.  I see 930Mbs up and down when wired in to their router.

I like the features i get with the deco - quickly being able to block a device from the mobile app is great - so am continuing to use it double natted with the ADTRAN router.

I also see roughly 250Mbs going through the Deco when wired or wireless.  I did have QoS with high priority devices set and removed all these.  Rebooted the deco and got 450Mbs on WiFi.  I thought I'd cured it, but testing with wired connection to the deco only got 250Mbs and wireless also dropped back to 250Mbs.  It seems that the higher speeds are available only very briefly after reboot.  

With AP mode, while it would sort the double nat, the issue is losing the parental controls.  Shame, as the Deco cured most of our WiFi issues throughout the house (although Ethernet backhaul is playing up again).  I'm going to have to look for a new solution.

Khanage wrote

Hi Why is my M4 capped to 250Mbps in router mode? It works perfect at 1000Mbps with the ISP router. I don't want to use the M4 in AP mode. My firmware is up to date

  This.

rotoq wrote

Update to this: have made the switch to using routers ISP and Deco in Access Point mode and can now get much faster speeds over wifi and ethernet, however I HATE my ISPs router and management console, so I only see this as a heavy trade off and short term test to see what headache I'd prefer: unutilised internet bandwidth or complex cumbersome network setup with poor config options

@rotoq 

  @McMike So i have been doing a bit more work on my network today  and I have a theory that seems to be standing up.  As mentioned before I was getting decent speeds (350, 400, 450Mbs) immediately after a reboot but these quickly fell back to 250 Mbs max.

Today I've also been looking in to the ethernet backhaul issue I've had and tracked it down to the switch that the 2nd Deco was attached to (big overcomplicated Cisco 48 port thing).

Replacing this switch I can now get ethernet backhaul working correctly AND my speeds through the Deco are much improved.  Via wireless I was seeing  520/518 while stood next to it, and wired via laptop I got as high as 440/885.

I upgraded my firmware this morning to the latest, but it didn't improve speeds, but it did move the QoS button back to the normal menu instead of its own big button.  I could see a priority device in there that had previously been deleted and deleted this again.  So currently, no devices show as High Priority (again).

My gut feeling is that wifi backhaul drags the speeds down and eternet backhaul resolves it and when I was rebooting earlier, the better speeds were before the 2nd Deco linked up again on wifi.

  @SpeedyG I had this problem as well (wonder how many years I've been paying my ISP for this high speed and never utilizing it). Almost paid $300 for a new deco system. Thank YOU SO MUCH for sharing your solution with us. This fixed my issue. I am beyond happy.

I have a Gigabit connection. Was getting 990mbps up/down directly connected to the modem with a cat 6 cable. QOS settings set to 1000/1000. But through my deco S4 I would be capped at an oddly specific and reliably consistent 245~250mbps. So odd. Nobody was able to help. This Google search brought me here. I went into my deco app, removed the device I had set to "high priority" and ran another speed test. I'm getting 800~850 mbps through the S4 now. Literally x4 the speed because this bottleneck was removed. 

A very embarassing bug that apparently TP Link hasn't fixed in 2 years. But then again.... I almost paid $300 for a new deco system. So maybe it's by design... 

I will try to give you credit if anyone else on the webs has this issue. Thanks again.