Well, do you have Android or Windows 10 running on device? Did you try to manually adding WI-Fi-information to device?
Sometimes the 5GHz router don't go on if you have a lot of neighbours, it is the iso-policy of routers.
Do you see the 5GHz band but cannot connect it? Try some apps that scans the air, also if you have possibilities to connect to another router just for "check".
Try to deactivate the routers 2.4GHz and test further.
Which router do you use and firmware?
Is the ROUTER setting set on channel width:
20MHz (2.4GHz) ?
40Mhz (5.0GHz) ?
or 80MHz (ac) ?
Thanks for your reply. Funny enough I turned on mi pad today and it's connected to 5ghz wifi. I haven't changed or done anything since. So, I guess this is resolved for now.