Are the longer or shorter antennas for 2.4G? I have Archer C50 with 4 antennas (2+2) and I want to orient them properly but I am not sure which two are for 2.4G Wifi.
The signal radiates from the side of the antenna, so best results are almost always with the antennas pointed straight up. They only swivel for shipping purposes and to look cool in product photos.
You can unmount the longer or the shorter ones and check for the wifi signal strengths to identify the corresponding antennas.
I have 2.4G devices at -5 meters from AP level, so I want to orient these two antennas to get better signal while keeping the 5G vertical. I will try to remove one type or cover them in aluminium foil 
to identify them
Find a picture of the inside or open it up yourself. The antennas connected to the MT7620 chip are for 2.4 and the ones connected to the MT7612 chip are for 5 GHz.
Looks like the shorter ones on the side are 5G
I did test with aluminium foil to confirm that