Have you tried switching off the 2.4 GHz band if possible on the router? That or forgetting the login details for the current connection to your WiFi on your phone settings?
Personally what I would do is download a WiFi analyser app like wifianalyzer open source from the play store.
I would then check I can actually see the 5ghz connection and how strong it is. Also I would see if it's clashing with other 5ghz signals. It's unlikely but I can recall a few years ago a neighbour and I both used 5g and what ever channel I selected for 5g his router would for some reason automatically switch to the same channel. In the end we fixed the channels So they did not clash.
2.4ghz will travel far further so it maybe all that can be picked up, as mentioned by Strampke.
Also have you checked in which direction your router aerial is pointing? While not always a major difference higher frequencies tend to be a tad more directional than lower frequencies.
Plus router aerials don't always spread the signal equally around (omnidirectional) or focus the signal in one direction (directional)
You may find depending on the type either the signal is being spread all around the router but not far enough or it's being all directed in a direction other than ideal for your device.
I would first play with the app and see no issue with picking up the signal.
If you can see a signal then maybe time to look at the settings on the router and the phone.
Have you checked by logging into the rosters admin pages with a pc pr your phone?
Are you sure no separate login details for the 5g or Mac address access set?
Does the 5g network have a different type of encryption enabled?
Is there actually a 5gz broadcast? Possibly the router in theory can have the technolgy but it is disabled either by the manufacturer, the particular firmware, or by a setting.
