In the past (https://forum.openwrt.org/viewtopic.php?id=39262) I have tried to get affinity working on a different router (WDR4300). No luck at that time.
Instead it looks like that it could work on the WDR4900.
In fact the iw sub-comand

iw phy phy0 set antenna <TX_mask> <RX_mask>

seems to work:

root@NOA-OGW:~# iw phy phy0 info  | grep Antennas
        Available Antennas: TX 0x7 RX 0x7
        Configured Antennas: TX 0x7 RX 0x7
root@NOA-OGW:~# iw phy phy0 set antenna 1 1
root@NOA-OGW:~# iw phy phy0 info  | grep Antennas
        Available Antennas: TX 0x7 RX 0x7
        Configured Antennas: TX 0x1 RX 0x1
root@NOA-OGW:~# iw phy phy0 set antenna 7 7
root@NOA-OGW:~# iw phy phy0 info  | grep Antennas
        Available Antennas: TX 0x7 RX 0x7
        Configured Antennas: TX 0x7 RX 0x7

Now I am back to usual question:
As we have three antennas handled by a three bit netmask and numbered 0, 1 and 2 (or 1,2, and 3), which one is no.1 and which one is no.2?
I would like to create a 5GHz backbone on a separate antenna and thus I need to know which one will get the directional external antenna.

TIA.