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How did you set up QoS? Post a picture?
You can use the DIR-810L as a 2ndary wired AP if you do this:Turning a router into an AP.
Are you wanting both the radios on the 645 and 810L to operate at the same time? If so, then you need to place the 645 on channel 11 and the 810L on channel 1.Farthest channels apart as possible. 5Ghz can be anywhere or unless other neighboring 5Ghz are around, try channel 48.