@David-TP  

My internet speed is 250 Mbps from speed test of Deco. QOS is closed. The speed of connected iPhone is 40 Mbps with main Deco, even if I connect iPhone to satelite Deco, speed reduced to 24 Mbps which is understandable. But Main Deco can not be closer to internet speed. I attached screenshuts.

  @David-TP 

I changed main Deco already. Because, I had cable connection with modem and I was using another Deco as main Deco. When I decided to use a modem with LTE (SIM card), I replaced with previous connection. I was using at that time a satelite deco because cable was near by another Deco. Speed was much more lower. Since it is possible to move modem with SIM card, I made a satelite deco as main deco because it is closer to living room.

I was not observed this speed problem before because internet connection was slow with cable modem. I am using modem with SIM card and connection fast. I realised this speed problem recently. 

About to test 2.4 GHz, I will do it and report you here. But I don't believe this is limit theoretical limit of 2.4 Ghz is around 450bps, I am near Deco and should be at least 200 bps.

In order to avoid interferences between modems, I switched of WiFi of LTE Modem. Because near to main Deco and connected via cable. If I use WiFi of LTE Modem, I can see more than 200 bps without problem with 2.4 GHz. I am sure because SSID of 5 Ghz is different for LTE Modem.