Just enabled 802.11r on my network. Two OpenWrt routers, one running 17.01.6 and serving as my Gateway (old router and already on the latest supported OpenWrt), and the 'wifi repeater' running 19.07.3 (this one seemingly has better coverage so I am using it as a repeater). I'll call the Gateway AP #1 and the Repeater AP #2.
Tested 802.11r with an old iPhone 6s and my current phone which is a Xiaomi Mi 9.
iPhone - doesn't need much testing. Played a video while connected to AP #1, while playing I moved to an area out of coverage for AP #1. Movie continued to play. And AP #2 showed it is now connected to the phone. Repeated several times, same result. No problem. Test passed with flying colors.
Mi 9 - repeated the above test. Video playing stopped, network got disconnected for a couple of seconds before AP #2 picks it up. Problem can be reproduced with the same test 99% of the time (the only time it seemed to have worked was probably just fluke).
Searched on the web, seems Android is notorious for 802.11r compatibility, and only a handful of Samsung phones claim to support 802.11r. So... not much I can do to fix this problem, right? My Mi 9 is rooted btw so if any commands can be sent to trigger the recognition of 802.11r I would probably be able to do it. But I use the WifiAnalyzer app on this phone and it already shows the expected "+FT" for the two APs.
Or will the addition of 802.11k and 802.11v help at all? I believe however my old router doesn't have this capability as it is running an old version of OpenWrt, thus an old hostapd.
Any help appreciated !