@Scope 

Go to Advanced > System Tools > System Parameters, enable NAT Boost, what it should do in turn is to disable QoS to see if QoS is slowing the connection. If that does not work, try disabling WMM.

@Tony Disabling WMM fixed the issue. Getting much faster wifi speeds on both 2.4 and 5 now. Thanks!

@Scope 

You are welcome.

Thank you for following up on the thread, appreciate it.

@Scope Do you know how I can get to the screen to turn off that option off? The WMM option.

@Mom_Paper 

Log into the router using a web browser. On the top of the page go to "Advanced", left side go to "System Tools", then select "System Parameters" left side.

@Tony - Thanks for the information.  What speeds should I be seeing over 2.4G on my cell?  I was seeing 10/30 before the change and I'm now seeing closer to 20/30.  Is that expected?

@EGY 

There is not a exact speed to expect, the disabling of WMM mainly aims to improve of what you already have.

@Scope I'm having the exact same issue. 

Thinking of scrapping the TP-Link router and getting a Mesh, definitely not TP-Link.. 

I have wasted enough time trying to fix the WiFi speed. 

Thanks for this. I just got the TP-Link AC1200 and was having issues with the wi-fi speeds capping at about 70-80Mbps.  When I do a test off of my ISP wireless modem, i get around 200+Mbps on my iphone 8+.  Disabled the WMM and it jumped up to about 140Mbps.  Not as fast as my other device, but a deffinant improvement.