speed is slow?
My UniFI 30/10mbps router is also using DIR 850.

mind telling me what kind of device you are accessing the router with? smartphones? ipads? desktop? laptop? etc

all my smartphones, ipads, PS4 & laptop can get around 27-30 mbps speed.
but sometimes my desktop pc can get up to 10-15mbps only. but this is due to my desktop is using Dell wireless adapter, which is known to have low reception problem.

what i did was, i tweaked some settings inside it, especially on antennas & channel strength, and i can get vast improvement. can easily get 27-30mbps now.