I recently changed out my router to a newer model (TP-Link AC1900) and am experiencing significant packet loss (30-70%) when using the 5G WiFi on my 2017 iMac Retina (2.4GHz WiFi works fine).   I have changed the location of the router and still have the packet loss.  Other devices such as an iPad Pro and older MacBook Pro do not have any packet loss issues.  I am testing over the LAN (192.168.0.1) in order to rule out any Internet connection issues.  I have performed multiple reset/restarts of my modem, router and iMac and nothing has changed.

When I hooked the older router (Archer C7) back up, placed it in the same location as the newer router and accessed 5G WiFi, I don't get any packet loss.  So the issue seems to be the interaction between the one iMac Retina and the newer router.

The main differences between the older and newer routers are three vs four antennas, and the new router having beam forming capabilities.

I prefer naming the 2.4 and 5G WiFi networks with the same SSID so that the best network is chosen and I don't have to manually select WiFi frequencies.  But since the iMac defaults to the 5G network if the relative signal powers are close, it always picks the 5G network and I start having the packet loss issues.  To avoid this, I have given the 2.4 and 5GHz frequencies different name for the time being.

Are there any configuration changes I can make to the router and/or iMac so that selecting 5G WiFi doesn't create problems?  Otherwise, I'll be returning the new router.