@Willem33 

I just had conversation on this forum in regard to slow speed on Deco X20, and after fixing an issue with their WiFi configuration that person is capable of getting 700Mpbs on iPhone next to Main Deco. You have much superior X90, you should be getting at least that, and not just 350/333Mbps. That conversation I had, perhaps it can be helpful start with the troubleshooting of your Deco WiFi: DECO X20 - Slow Speed

In regard to UNKNOWN devices: if you made an upgrade by deleting and recreating WiFi mesh, instead of just adding and reassigning X90 as Main Deco, it is possible that triggered MAC Address Randomization feature in devices such as iPhones. You'll need to turn it off on every device, for SSID of your home network. See post #2 in this thread: UNKNOWN has joined the network.

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To help you with Deco WiFi performance issues, please tell the following:

Do you run your Deco mesh in Router mode or in Access Point mode?

What device (brand, model) do you use to run speed tests and does that mobile device support WiFi6?

Do you have device such as laptop or desktop with gigabit Ethernet that you can hardwire to different Decos to run speed tests? That will be very handy in troubleshooting.

@Alexandre. 

Thanks for your response! let me answer your questions:

To help you with Deco WiFi performance issues, please tell the following:

Do you run your Deco mesh in Router mode or in Access Point mode? Router mode

What device (brand, model) do you use to run speed tests and does that mobile device support WiFi6? I run Ookla Speedtest on my iPhone 12 Pro.

Do you have device such as laptop or desktop with gigabit Ethernet that you can hardwire to different Decos to run speed tests? That will be very handy in troubleshooting. No I don't work on Wifi only. My LG Qled TV has a built in speedtest and is wired to my X90; there I only got a shocking 80 / 35 Mbs back :-(

@Willem33 

I haven't checked specs for that brand of TV, but every other Smart TV I know has 100Mbps network card, not gigabit. They don't need gigabit speed, you can comfortably stream at 4K with 100Mbps wired. Also, because TV CPU is not of fastest and TV RAM is limited, that might impact results of speed test running from TV. Meaning, 80/35 Mbps on Smart TV would be OK and also TV is not the wired device I would use for speed tests.

I would suggest starting with Main Deco troubleshooting, just like I did here: DECO X20 - Slow Speed 

I gave two recommendations: one to check and if necessary increase the value of QoS and another is to run speed test with WiFi 2.4GHz turned off on Main Deco network. Please try them. Just like person I spoke to, you should be able to achieve at least 600-700Mbps on your iPhone 12 if your Main Deco X90 is properly configured and band steering is disabled, which is when Deco Main Network has only 5GHz band enabled.

Please try that, as I explained there, and share speed test results.