Hi Bud, I don't know if this might help, but I wrestled with this for over a week on an Archer C8, maybe it applies to the Archer C7. New 500/50 service, and no matter what I did, I couldn't get more than 330 down through the router.  Direct wire to modem, I'd get 550+ down. I strangled the router's settings in every way I could think, QoS, Nat Boost, double checked update firmware, hard reset, soft reset, and on and on. Then this:  In the router's admin screen, I turned off Nat Boost, disabled all the ALG checkboxes, and enabled QoS.  Checked results, still slow.  I turned all the ALG services back on, then enabled Nat Boost again (takes a bit of time for router after you enable that, makes you wait.  Save my settings, back to the internet speed test, and *boom* -- all problems solved.  All the wired-to-router computers were getting 550+ speeds from the internet. Go figure. I can't say that's what "fixed" it- all I can say is it's not broken anymore.

@StevenRN Did it fix for good?

One thing I noticed with TP-Link routers is that over time the slow speed comes back.

I have a C5400 and face the same issue.