I've looked it up a bit, but I can't get it to work either.
The official TP-Link website states that it only supports 2.4 Ghz:
https://www.tp-link.com/de...However, the manual for this device states that it can be customised:
https://static.tp-link.com/res...However, this is a manual for an older version.
My guess is that this function has been removed from the new iterations of TP-Link M3750. Hardware and/or software wise.
I have adjusted the specifications at Digitec.