I have two deco x50 units, the one in the 1st floor was able to connect to the main unit in the 2nd floor with 2.4/5 Ghz, now it only connect to 2.4 GHz, how to make it able again to use 5GHz, and is

I got a 2 pack Deco X50 to work as access point to expand my wifi coverage. My House consists of 2 floors. In the 2nd floor, I plugged one unit with the modem and it works good with the full speed rea