The ssid for 5G is not visible. I have read many threads reporting the problem - but no solution. It is marked as visible. One odd thing I observe with LUCI interface is that both 5G and 2.4G seem to show a driver name with “bgn” in it - the one for 5G band should be using a 802.11a driver.
Problem seems to have been around for a while and read many threads on many different router models but dont see any solution.
My router is MT-1300 where it used to work fine (might be with an earlier firmware!)