This has been the case since I got the router last year. It has gotten a couple of firmware upgrades but the issue remains. How can a $20 Xiaomi offer better performance? It is very embarrassing too since I am the go to IT person for people nearer to me.

@Kevin_Z the settings i have tried a bunch.
- band: currently it is ac only but i have tried all other options
- width: i have tried 80, 40, auto
- security: i have tried wpa/wpa2 and currently wpa2/wpa3 
- airtime fairness ans mimo all permutations i think

firmware version is shown in the picture above.
 

For the first time I was able to connect my iPhone 12 Pro Max to the 5GHz network. But I had to restart the phone.

I could connect the MBA 2018 after restart too but I have been able to do that before.

The thing to check now is if the iPhone 12 and MBA retina 2018 are able to maintain the 5GHz or at least can see it (without requiring restart) if it disconnects.

I think the key might be channel 48? I think I never tried that before.

But the iPhone 12 connecting to 5GHz (and I hope it keeps connecting at least after restart) is a huge win for me for which I can't thank you enough!

@Kevin_Z Well, it's better than nothing i.e. I have to restart the iPhone after I get back home to connect to the 5GHz. Previously, iPhone 12 Pro Max didn't even see the 5GHz. But both the MBA 2018 and iPhone 12 can't see 5GHz after I disconnect from it (because I'm out or want to connect to another network) unless I restart them (while the older devices iPhone X, iPad Pro 2015, etc have no  problems). However, when I run Wi-Fi scanning using built in diagnostics / AirPort utility, they do scan it but it's not visible in connections.

@Kevin_Z Another update. TP Link app has the feature to  generate QR code to connect. My iPhone 12 can't see the 5GHz network but scanning the QR code lets it connect. What is going on?