@maciejp Hello, 

As range extender needs to receive data from the router and then forward it to the connected clients, speed will decrease during the transmission, especially via wireless connection.  If you are able to wire  the range extender to router, please set it up into AP mode and see if speed gets better: 

Router--- < Ethernet >---RE (AP Mode), please switch RE to AP Mode : How to configure the range extender as an access point (new logo)?

If you could not run a cable between RE and router, may I know at RE's location, how much speed did you get from main router's 5Ghz WiFi? 

200-250Mb/s, is this the speed on RE605 5Ghz Wi-Fi network? 

@maciejp  Hello, 

It's not a good idea to connect RE to router via cable & wireless at the same time. It  may cause a loop between router and extender which leads to a network storm in the network after certain time. 

Without oneness,  wireless clients could still  roam between router and range extender (most new wireless device support roaming), its only not that fast as oneMesh.

I assume you have to make a choice between AP Mode or OneMesh, it is rec use it in AP mode unless you need to always walk around in video call.

@maciejp Hello, 

This weird behavior on tether's wireless client is probably caused by your uncommon network topology. 

You could use RE650X as range extender only and disconnect the cable for a test . 

Please close the app and reopen it after few mins ( client list  takes some time to refresh)after you disconnect the cable, and see the issue will be gone or not. 

@maciejp 

Thank you for your update, unfortunately we did not hear a plan to support oneMesh in AP Mode, only RE mode supports Onemesh feature.

When RE (AP) does not use OneMesh, clients could still roam between router and AP, but not as fast as oneMesh. I suggest testing AP Mode for certain time. 

If stop using OneMesh does affect your daily network activities, you could consider switching back to RE mode.