Low bandwith using WiFi 6 AX

Hello engineers !

I have a question about configuring router OS 7.13 with CAPSMAN and bandwith located to WiFi 6 AX. Question is basic: I have 1Gbit WAN. I have this speed accorded to ethernet cable but at WiFi 6 AX I have speed located to 250-350 mbit/s.

I cannot on CAPSMAN give a MTU (putted 1500 is erasing after Apply configuration) and maybe there is some misconfiguration I made or made-not during configuration by CAPSMAN MODE?

Could You give me a thing to check out for ?

Well for start best thing is to export configuration of your router and tell us what device are you using as CAP ?