I'm sure you have seen this post before, but I found it just a few weeks ago... I had success using this setting with no disconnects on a WRT3200ACM. The script disables AMSDU. Stability for >2 weeks on the 21.02 release. Without this, I had the same issue everyone else describes, with iPhone X, iPad, etc. showing connected to WiFi but having no response for several minutes.
I'm not sure what this points to other than a possible incompatibility with iPhone and the firmware, or maybe a kernel issue with handling this AMSU setting.