There won’t be any data loss just because you’re running them through a switch - it’s a normal setup. As long as the switch is gigabit (which you said it is), you’ll still get full bandwidth from your main router to the other Decos. So your X10 and X50 connected through the Cat-6 cables will work fine, no speed reduction compared to plugging them directly into the main unit

  @Vendex Mother or main router speed

  @Vendex 

  @SayeemAhmed 

As I see, on the main unit your phone is connected on wifi6, what band is connected on the second unit, can you check that please?

  @SayeemAhmed 

Hi, can I have the model number of the gigabit switch?

It seems the Ethernet negotiation speed is somehow capped at 100 Mbps. 

Apart from the two satellite Deco units, do you have other Ethernet devices connected to the switch?

By the way, is the wireless speed from both satellite Deco units less than 100mbps?

 It is also suggested to change a new cable to connect the main Deco X50 to the gigabit switch.

Thank you very much.

Wait for your reply.

  @David-TP switch... 

  @Vendex Check to Wifi-6

  @David-TP hello...!?

  @SayeemAhmed Hi, thank you very much for your time and patience.

I see from the SN number of the switch that it is about 4 years old. Do you have any wired laptop that could be used to replace "the second floor Deco X10" and connect to the same switch-TL-SG1005G to test the internet speed on the laptop?

And satellite Mesh units work the same. You can also switch the location of "the first floor Deco X50" with "the second floor Deco X10" to see if the slow speed follows the Deco X10 or stays in the same location.

Many users use network switches to establish Ethernet Backhaul. This topology itself would not affect the Mesh speed. Usually, an aging Ethernet structure or a broken cable might reduce the network speed.

Thanks a lot.

Best regards.

  @David-TP I reset the X10 and reconnected it, which fixed it, but I tried the same thing with the X50, but it doesn't work at all. Is this the reason why it's like this? I can try changing the Cat-6 cable, but I have to wait to do that.