Devices (iPhones especially) disconnect from wifi daily

I've been experiencing a strange issue with our wifi – some devices (notably iPhone 13 Pro and Homepod minis) lose access to wifi about daily. I am not sure how to describe the situation properly. The devices themselves report being connected to wifi however they cannot actually access the internet (or even the lan). One evening we noticed our phones stopped connecting to the internet at midnight exactly.

We can usually resolve the issue by power cycling the affected devices, but sometimes I have to reboot the wifi AP, altho this has been less necessary since disabling AX mode. Its worth noting that other devices such as my laptop do not experience the same issue and can continue to use wifi even when the iPhones are having problems.

I have played around with config settings that seem to have made it less bad (dropping from AX to AC mode) but I am out of ideas. I'm asking here for 1) tips on how to investigate this issue more thoroughly (detailed logging, etc etc) and 2) any similar experiences?

My setup is as follows:

Fiber/DSL modem ↔️ openwrt gateway (RPi 4) ↔️ switch ↔️ ubiquiti 6 LR (running openwrt) as dumb AP

Everything is on 21.02.3. The only thing I can think of that is especially peculiar about my situation is that my ISP does not support IPv6 properly (Centurylink in Seattle) but I don't think that should affect the wifi issue.