iPhone Xs stucks at 20Mhz in 5Ghz


I have a problem with my iPhone Xs and 5Ghz Wi-Fi in OpenWrt, which drives me crazy.

iPhone Xs as only one 5Ghz client.
2.4Ghz works well (as it could).
Router (in my case this was TP-LINK Archer C6 first, then Xiaomi MI Router AC2100) in AP mode with 2.4/5Ghz wireless networks. One or separate SSIDs, no difference.
My iPhone works well at first, got 90/90 of my 100Mbit LAN via 5Ghz (40, 80Mhz channel wide). Then, after some random period of time (I suppose after iPhone goes to deep sleep mode), my iPhone "stucks" in 40/60Mbit via same 5Ghz wireless, same distance to AP, same channel and signal strength.
What does not help: Wi-Fi turn off/on on iPhone, any manipulations with wireless settings in OpenWrt (WMM off/on, disabling ACK timeout or diassoc, any settings), even router reboot does not help.
The only thing that working is rebooting iPhone completely. After reboot wireless speed returns to normal and problem repeats again.

I've mentioned TP-LINK Archer C6 above, this problem was with this router as well as with my current AC2100 (I rid of C6 because of this).

But. I also have MikroTik hAP AC lite, and this problem does not occur on OpenWrt neither than RouterOS.

I appreciate any help: how can I diagnose this strange behavior?

Also tested on friend's iPhone 7 and he have the same problem.
OpenWrt version: SNAPSHOT r15540-20a7c9d5c9

Problem occurs in SNAPSHOT r15679-43ff6e641e as well.

So, no one using iPhones with MT7615E here?