MR2100 - can't print to printer or browse Printer webUI on 2.4Ghz (but can ping) from 5GHz

An #MR2100 has a printer connected to the 2.4GHz wifi
If a computer is connected to the same band, it can print fine, but if the computer connects to the 5GHz band it can't print to the printer. It also can't access the printer web interface through a browser when connected to the 5GHz band.
But you can ping the printer from the 5GHz band (WTF?!?!)

The MR2100 router dashboard also shows the printer connected to the 2.4GHz wifi band.

Why can't the router pass HTTP and printing traffic between the two bands, but PING (ICMP) works fine?