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...!?