Good afternoon! I have a Mikrotik Hap AX³ model that doesn't show the 5GHz frequency. I've already checked the country and channel, and it doesn't appear on my cell phone or laptop. I have another of the same model that didn't require any configuration, and it does appear for me. Any suggestions on what the problem might be? I'm almost certain it's a hardware issue with the device itself.
Check used frequency.
Some clients can not handle the higher UNII-3 channels.
Better, set the frequency yourself so you know where it will be.
I've already set the frequency, and I've changed the channel and band, and I've also configured it to not use the DFS and UNI-3-4 channels.
I even tried changing the network type to private; it appeared, but the range was low and there was no performance.
mostly hardware issue with the device itself，try Factory Reset！
Do /interface/wifi/export of both devices and compare side by side. There must be a difference.
And this rules out a lot of possible HW issues.
So most likely config.
Either you post config here for review, either you do as suggested by infabo and compare config yourself.