@sloop The 2.4ghz and 5ghz use two different physical RF radios, so no, you can't create an exception like that. I would try to create a new SSID that only operates on the 2.4ghz band, make sure it setup to use the same VLAN (I'm assuming you are using the default one), and connect only your printer to this SSID. For example:
Sloop (5ghz only SSID)
Sloop-2.4G (2.4ghz only SSID) - connect your printer here