RBK50+S. How to allow only n and block a/b/g/ac

It would help to know what issue this is expected to address. 802.11n is a 2.4G standard that is "backward compatible" with earlier standards b and g. 802.11ac is a 5G standard that has nothing to do with 802.11n.

There is not much point in having an Orbi and not allowing 5G. Even if the Orbi was replaced with a 802.11n WiFi, that device would accept b and g connections. 

Smart connect and WiFi Mesh systems are all automatic now days. Handling of WiFi modes on Orbi is all automatic and handled between the Orbi and the connnecting devices. Selecting of different modes isn't supported on Orbi. Other NG routers you can do this if you disable Smart connect and set the Max Speed Rate supported by the router. However NG hasn't supported any kind of specifc modes to be selectect by the user. Just which speed rates maybe used is all. Orbi it's all automatic.