When I play on my PC (hard wired) on my regular PC monitor, I have a near flawless experience. It looks and plays great with virtually no lag.
When I switch over to my monitor # two, which is an HDTV connected via HDMI to that same PC, I almost always get disconnected immediately and it tells me to reconnect after my connection improves. Of course it's extremely laggy and very bad pixelation. Now why would a monitor make a difference? I have my HDTV set the game mode already. I'm not really sure what else to try.
I've heard reports in the past about issues with second monitors on some machines. It's possible that port is being driven by on-board video instead of a discrete GPU, which ends up not managing to make hardware video decoding work, overloading your CPU attempting to do it in software (which ends up looking like a network problem). So, I would be digging deeper into what video connections you have and where exactly they're coming from on your PC. You may have better luck by having only the HDTV connected when you play.
Thanks for the response. My PC's built-in video card is turned off in BIOS. Both the monitor and tv are plugged into each one of the HDMI out on my Radeon RX 570. But I did try making my TV the only output, but I still got errors about connection.
I forgot to mention: I play destiny 2 exclusively via a controller. But neither the controller nor mouse and keyboard make a difference. It is just a bad experience on the TV only.