I set wlan2 on my wap-ac to 5ghz-a/n/ac

A non-AC capable client cannot see its SSID.
On MT wlan status window, it is running channel 5180/20/ac

All ok if Band is set to 5ghz-A/N

How should I configure wlan in a wireless environment where older devices are present ???

Play with extension channel.

What are your clients?

I'm wondering if it is more a problem with the channel chosen vs the A/N/AC problem.

Yes , it was an unsupported channel choice..... not an A/N/AC problem...