We have a new G3100 and E3200 setup. Previously had the G1100 with two extenders. The Wi-FI speed is inconsistent, to say the least. SON is turned off because the system kept connecting directly to the 2.4 GHz channel even if it was a weaker signal! With SON turned off we now have two 5 GHz channels to choose from and I have named them 5.1 and 5.2. When I freshly connect to either one of the 5 GHz channels available and check the speed I am connected to Wi-Fi 6 at 1.1 Gps and a minute later without moving anything, it drops to 144 even lower! Both the G3100 and E3200 have the 18.104.22.168 Firmware version. I know the device is connected to the E3200 through checking it in Devices. I have also checked the signal strength and cross-over for each channel and we are connected to the best option but continue to get lackluster Wi-Fi speeds even when only 10 to 20 feet away from both the G3100 and E3200! The Wi-Fi coverage is supposed to be better but is about the same or less than the G1100 with Extender and the Wi-FI speeds are not there. We do see Wi-Fi 6 on our connected AX devices but at a loss when the Wi-Fi speeds are so bad even when taking into account all the factors which might impact Wi-Fi speeds. We are using WPA2, transmit power is 100, and running the two 5 GHz channels in 802.11 Mode in Compatibility Mode (802.11a/n/ac/ax). The cost to upgrade and not have any benefit is disappointing.
The E3200 is connected via COAX on the second floor of our home and is shown on the Router configuration page. Would it be better to connect the E3200 without the COAX? Are there any other settings in the configuration of both the G3100 and E3200 which would increase performance?
Using coax backhaul is the best way.
I could not reproduce your scenario of having near gigabit link speed to suddenly dropping to 144 Mbps.
Could you install the Iperf3 speed test utility on one computer wired gigabit Ethernet and on your WiFi6 capable phone? Then run the Iperf3 for 10 minutes, can you see when you would get a speed drop?