I'm having issues connecting an older kindle (2.4GHz only) to my network. I have tested the kindle for WiFi issues, but it connects fine to my phone hotspot and also fine to the bundled TalkTalk router at my mum's (combined 2.4 and 5GHz bands), so it doesn't seem to be an issue with the Kindle itself.
My setup at home consists of two Linksys MR8300 routers that are hard linked with the WiFi channels using 802.11r fast switching via OpenWRT 19.07. This has worked fine for all devices I have tried to connect, apart from the Kindle. I initially thought it might have been the fast switching feature that was not compatible, but I setup a test 2.4GHz SSID with no security and no fast switching, but the device still failed to connect.
Just to add to what @faser mentioned, I’ve had problems with older devices when management frame protection is enabled, even when set to optional, so you might try disabling it too if you have it enabled.