I have a new (one month) Deco mesh network, X10. One is connected to the router, two others connect to that unit with ethernet backhaul. One unit only works on the 2.4ghz network and has much lower performance, 30% of router speed whereas the others are very close to full speed. I have verified that this is an issue with that unit - leaving everything in place (power cables, connections, switch, ethernet cables) - and swapping units around, only that unit has this problem. I have swapped everything around several times and it is still that one unit. A scan shows no interference.  While the speed is generally sufficient, I sometimes need to use a VPN to access certain sites in my home country and the speed is sometimes too low.

I can't find a setting that would change just one unit, but maybe I'm not looking in the right place. Or maybe it's a defective unit.