OK, this makes sense. As you can see the 2.4 GHz radio that you have set-up on the Netgear router has 11 WiFi devices on it, which isn’t a good thing. These devices would perform better on your 5 GHz radios.
It is also competing for the same space as your other two 2.4 GHz radios. My recommendation again is to disable this 2.4 GHz radio entirely.
I have DSL 12-16mbps. Should I be using QoS?
If you see an option to enable or disable QoS, it should probably be enabled. QoS simply prioritizes certain WiFi traffic such as audio in a video call.