Since most phones or even pc's don't seem to show the band info in their native WiFi hotspots list, an easy way to diagnose these issues is by using a WiFi analyzer on an Android phone (made after 2014) which you know for sure has both 2.4 and 5Ghz bands available. You can use something like WiFi Analyzer (https://play.google.com/store/apps/deta ... nalyzerpro
), or any other popular analyzer in the PlayStore which shows the hotspots categorized by bands. It's a great tool for trying to solve all kinds of WiFi issues.
If your phone can see the 5G signal, yet still unable to connect, then there's likely an authentication error. First, click to forget the router's current hotspot name if that name has been registered in your phone at some point in the past. When you click on the router's hotspot name on 5G, do you get the a prompt asking for the password? Now, this is important...
If not, then they have a total protocol breakdown, but if it asks for the password then you're either entering the code incorrectly or it has been set to some non-standard characters including the hotspot's name. Make sure to use a simple hotspot name like "HOTSPOT", and for the password simply enter 12345678
If none of these suggestions work, then it's possible that the router either has a defective 5G WiFi chip OR it's 5G antenna connections were soldered on incorrectly or with a short (seen this issue before and corrected it by resoldering the antenna wires properly). Last but not least, you could also play around with the 5G WiFi's power settings, fragmentation length, CTS/RTS Threshold, and Preamble Mode (Long/Short).