Hi there,
I was without 5 GHz WLAN ca. 1 day after upgrading to iOS 14.4. 2.4 GHz still worked well on iPhone XS and iPad 8.
What I did:
(iOS units) removed both the 5 GHz and the 2.4 GHz WLAN network from Wifi settings
dis- and enabled 5 GHz WLAN in router settings
rebooted router
(iOS units) dis- and enabled Wifi
(iOS units) added the 5 GHz and the 2.4 GHz WLAN network in Wifi settings again
...and it works again. Will see how long. ;)
/Cheers