And actually, clients should (and will) do that themselves, with less issues than trying to control it from the access point.
It has been a long time since I saw a client that by default preferred 2.4 over 5 GHz connection.
Amazon Echo devices do this all the time.. At least 1.Gen ones that I have.
The comparable Google devices do it OK. Your device may be too old, it will probably get confused big time by 802.1r/v/k and maybe even by bandsteering.
It is better to just ignore this, it is not that important for that kind of device anyway.
I don't think it is still an issue for more powerful devices like laptops, phones, tablets etc.
New version 7.1beta2 has been released in development RouterOS channel:
http://forum.mikrotik.com/t/v7-1beta2-development-is-released/142356/1