@GreenThreat 

On a traditional router like the Archer AX55, it is.  I would assume you are using Smart Connect.  With this feature enabled the bands will share a single SSID and Password.  The router will also do all of the network assignments.  This includes the band a device is assigned to, the channel the bands use, and the channel bandwidth.  If you are making use of OneMesh then this feature is unavoidable but if you are not you can disable it and set two separate SSID, one for 5GHz and one for 2.4GHz.  Just go into the router's UI.  Click on Advanced >  Wireless > Wireless Settings.  At the top of the screen, you should see Smart Connect.  Just disable the feature, set your bands and that is it.   

  @Carl Thanks for they reply, however, this is not quite what I am looking for. I'd like to keep both bands on the same SSID, just force a single device to always use 5GHz, the rest should always be automatically assigned one of the two.

  @GreenThreat 

Unfortuently at this time that is not possible.  But we will add it to our request list that we send monthly to our development team.