Netgear AC1750 extender very slow wifi speed

Hi,

Before starting I must say that I already spent 3h on this problem and tried averyting I could find on this community. I also spent 2h with 5 support agents, each time being disconnected and it's just a nightmare... And Im computer ingeneer so I really really dont know what to do anymore or how any less technical profil could get this to work.

This is the basic problem context: when connected to 5G wifi directly to my router, I get 300 mbps. 

When connecting to the Netgear extender on 5G I get maximum 20mbps.

In order to exclude the range from my tests, I test everyting in the same room so extender is few feets from the router. I tried also connecting the extender on the floor where it should be and the problem remain the same.

What I already tried:

- 2 modes : separate name and one name for the wifi extender. 

- different wifi channels. I tried, 40, 120, 140, auto... I cannot go more than 140 with my router with 20mhz bandwith. WIth 80 mhz bandwith I can get to 112

- I tried using software to check the available wifi channels.. ALl those channels were available and I dont have a lot of wifi networks aroudn me

- I tried downgrading the firmwere from the latest one to the .100

I dont really know what to do anymore. My router model is the SFR router since I am living in France.

Can please anyone help me? 

I am quite desperate.