Thank you so so much for posting this. This worked for me.
Also thank you @Netduma Fraserfor the assistance in this thread.
Cheers!
Edit: I noticed after changing this setting a IoT device was not working. Now that I'm aware of this fix I decided to disable it to get the IoT device back online and troubleshoot from there. I expected RCS to stop working once I disabled the router setting ... however, I've undone the fix, restarted the router and my iphone, and RCS still works! I'll update if it reverts back to not working but I wanted to share.
Edit 2: Issue returned, but disabling QoS fixed the issue for good.