@Raivis 

Hi，

Could you please help me check the following information:

1. Have you tried to reset one of the M4 and add it back to see if the speed is improved?

2. Please check if the QoS, parental control and antivirus have been disabled or not;

3. Have you checked the Ethernet speed from the M4 to see if it is still capped to 50m/s？

4. In case the cable Is aging, could you please also replace a new cable to have a look?

Finally, if still capped to 50m/s on the satellite M4 units, could you please send me a speedtest result and also the link speed on this device so that we would forward your case to the senior engineers for further assistance.

Link speed on wireless device: https://www.tp-link.com/en/support/faq/2265/

Link speed on wired device: https://www.tp-link.com/en/support/faq/2899/

Thank you very much.