@Kamark 

Hi, the first Deco XE75 Pro needs to connect to BE9300(is it Archer BE550?) via an Ethernet cable.

To avoid double NAT, it is suggested to set Deco into access point mode:How to set up Deco to work in Access Point mode

(If you don't want them broadcast separate SSIDs, please try to disable the Wi-Fi on the BE9300).

Furthermore, If you just purchase TP-Link BE9300,  I think Deco BE65/BE65 Pro would be better to work as the main router since they both belong to the Deco family and could establish a Mesh system together.

Thank you very much and best regards.