@Ian-R Thanks TP-Link for your response. I am now quite perplexed.It seems that the disabling of the 2.4 GHz may have been irrelevant. First, I checked my PC's Ethernet adapter and it shows a speed...

Simple configuration: Ethernet cable from Broadband modem to main Deco Ethernet cable from main deco to PC Two other Deco boxes servicing various wifi devices throughout house "Test Internet Speed"...