Sorry for bumping this old topic. I have exactly same issue with OP. Tried everything, from downgrade to 1.10, flash all kinds of firmware version via recovery, reset to factory default twice, turn off smart connect and yet this problem of 5ghz missing still persist.
Also, WPA3 creates connecting issues to iphone and making two SSID (2.4 ghz's SSID, obviously) appears in Android. Selecting wrong SSID from both of them will show wrong password. This problem easily fixed by turning off WPA3, but it defeats the purpose of having more secure connection.
I have DAP-1665 too as an AP for my home office and the connection is bad, sometimes the ping spikes up although no interference detecting, and broke my work VPN connection. Replacing back my old 2008 TP-Link and never have this issue anymore. Needless to say, I'm kinda disappointed with D-Link products.
The only one works good is my DWA-171, which crashes my PC before upgrading the firmware to accept WPA3 from DIR-878, which I turned off because issues above.