mini mac late 2014 wifi/bt card is there an upgrade dual band card

There are no upgrades for Apple AirPort wireless cards for the 2014 Mac mini. You might find a USB-3 solution from a third-party but verify it is plug-n-play, without installable vendor drivers.

All Apple Macs have supported both 2.4 GHz and 5 GHz WiFi bands for probably a decade or more.

However, the various WiFi standards that utilize those bands have changed over the years. For the 2014 MacMini, it supports the 802.11AC WiFi specs and older.

So, if you have a WiFi router that is 802.11AC, your Mini will work just fine. Even if the WiFi router is newer AX specs it will still work just fine. It just won't achieve the higher AX speeds.

As far as the ultimate connection and download speeds, that is extremely environment dependent, i.e. you will almost never ever achieve the routers max speeds.