@MacPhly 

Hey

My thoughts are as you are connected via LAN to one of the DECO nodes, the backhaul to the main node where the ISP router is attached is over the WiFi mesh, likely the 5ghz channel.   Depending on the distance and connection of the DECO nodes this may be the section that is slowing your connection, I would near guarantee it.

If possible connect a laptop via LAN to the main node deco and test again using the cable, likely will be a full speed.    or take a mobile device and ensure its connected to the main deco wifi, test using speedtest.net...  go to the 2nd node, reconnect wifi and retest   -   Likely find this drops on the 2nd node

Mesh topology is handy,  but it does suffer the problem of WiFi interconnecting and therefore speeds will be an issue as you get faster.   From experience 300mbps would be the absolute max that a 2x2 AC backhall could support in a good scenario, add a few more nodes and device this will drop massively.  I believe this is your problem.

Honestly, for 300 down WAN connection you should be considering either a cable connection or worst case a WiFi6 mesh (the X range), it may be you are pushing the m5 mesh to its max.

@MacPhly 

Ah the old strange hidden settings!  Been there scratching my head so many times!

Glad you got sorted in the end