Builds from r40723 to r40900 are considered bad firmwares on TP-Link Archer C7 v2, because of enabling QoS settings will cause random reboots, and make the router unusable.I hope someone would create a ticket for that
What you are describing is pretty much the same for my D-Link DIR-859 ever since Cake qos was implemented(build r40634+). I hope Brainslayer will address this problem soon as its been going on for a while now.
I can't flash to any of the new builds due to the fact that there is a high risk of bricking my router based on my experience with builds r40634+.