@Ajmal_T I would recommend unmanaged switch. TP-Link recommends TL-SG108 for 8-port unmanaged switch. I like your choice better, because TL-SG108E has LED lights that show link speed - very useful...

@NikSf153 You will not need to add a Deco. This is how you new network layout will look, with Deco mesh running in Access Point mode: If you have enough Ethernet ports on new router you are going to...

@NikSf153 Depends, what is your ONU: device with just one Ethernet port, or is it Gateway (modem/router) type of device? If it is Gateway, you can switch Deco mesh to Access Point mode, but if it is...

@NikSf153 You will need to connect them like it is on first or second diagram, with Main Deco connecting directly to ONU and switch or Satellite Deco - to second port of Main Deco.

@NikSf153 Below is the picture from the TP-Link document I recommended you. See the last diagram on it, which is crossed out:

@NikSf153 Please, clarify for me, is this how your Main and Satellite Deco connect to the switch: