The culprit is likely dodgy 2.4GHz drivers. Archer C7 OpenWRT port never really worked well on 2.4. There are many threads about this. One of them: Archer C7 V2 - Massive Problems
Basically, 2.4 works with few device but if you load it enough it will start dropping packets or stop sending packets but stations are still associated. This persists until reboot.
Disable 2.4 and use 5GHz only.