Ah, let me see. I have the same problem connecting 5G WiFi. I am using a good brand WiFi router with 2G and 5G option. I set up at home both a 2G network, and called it my2Gnet, and another 5G network, my5Gnet.
Case 1 - Rpi connected to my2Gnet, with and without wired Ethernet connection.
No problem at all. I always see the "Fan like arcs sort of radiating
upwards with a dot a the bottom".
Case 2 - Rpi connected to my5Gnet and also wired Ethernet
Problem - The "Arc fan" icon only stays for about two seconds, then
changes to "Up down arrow", then after another two seconds, back to
"Arc Fan", and the cycle repeats forever.
Conclusion - I googled and learned that the "Arc fan" icon means connected to WiF, and (b) "Up Down arrow" icon means connected to wired Ethernet.
Case 3 - Rpi connected to 5G my5Gnet withOUT wired Ethernet
Problem - The icon repeat cycle becomes "Arc fan" and "Double red
arrow", each for two or three seconds.
Conclusion - I know "Double red arrow" means disconnected. So now it is (automatically) repeatedly connected and disconnected.
Case 4 - Rpi NOT connected to anything
Problem - "Double red arrow" for ever. Of course this is not a
problem, because I intentionally do not connect Rpi to WiFi or Ethernet.
Root Cause of Rpi Connecting to 5G WiFi Problem
I googled and found that Rpi buster is NOT responsible for the problem. The cause of problem roots back to her ancestor Ubantu who made a mistake or "sin".
Rpi has inherited the original sin.
So my big apology for wrongly accusing the lovely and innocent Rpi baby. :)
See references below.
(1) WiFi signal icon switches to Ethernet icon (up down arrows) after Suspend? - 2017dec
(2) How to restart network on Ubuntu 16.04 Xenial Xerus Linux - 2016apr
(3) 5GHz WiFi keeps disconnecting on Ubuntu 18.04.3 LTS - Ask Ubuntu 2019nov
End of answer