Asus RT-AC5300 poor WiFi on 5G-2

I would like to have a question to the WiFi experts:

my 5G-1 band gives max 866 MBit/s,

my 5G-2 band just offers me a max of 173,3 MBit/s

I'm using Merlin's last avalable FW because last available on (ASUS RT-AC5300 Firmware version 3.0.0.4.380.1355) from Asus didn't work on 5G band, 5G-2 didn't come up.

What's wrong with this 5G-2 band?
What could be a reason for this poor WiFi? I'm using my Lenovo T450s near the AC5300 router.

thanks

manilla