Explain airodump-ng 5ghz

Regarding the airodump-ng 5ghz - You need to make sure your capture card is capable of every flavour of signal modulation that the target devices can use. Your AWUS051NH is a a/b/g/n device. If your target AP and client (the devices whose traffic you want to capture) are 802.11a-, 802.11n-, or 802.11ac-capable devices, your a/b/g/n card won't be able to capture their traffic.

Even just looking for, say, "802.11ac" isn't enough, as there are variations of 802.11ac that not all devices support: 2, 3, 4 and more spatial streams; 80MHz, 80+80, and 160MHz-wide channels; MCS 8 and 9 (256-QAM), various channels that are legal/illegal in different countries, that equipment designed for different countries may or may not support, and more.

You must also make sure your capture device is positioned "in between" the target AP and the target client. That is, where it can receive transmissions from the target AP at least as well as the target client can, and where it can receive transmissions from the target client at least as well as the target AP can. You also need to make sure your interface is in both monitor mode and promiscuous mode (those two settings can be orthogonal; don't assume that monitor mode implies promiscuous mode on your hardware/driver), but it sounds like you already took care of that.

Uncertain if this will help or is entirely correct but after some google searching this is what I arrived at.