@mariomontes 

Hi, the Deco Mesh system is not the same as the OneMesh system, and they could not be used to create a seamless mesh together. Check the story we posted to know more about Deco mesh and Onemesh:

https://community.tp-link.com/en/home/stories/detail/1322

This article also offers some basic QAs, you can have a check:

Several questions about the OneMesh and Mesh

If the RE330 is still in the return windows of the reseller, you may check with the shop if this can be upgraded or replaced with another Deco.

If you cannot return or replace the RE330, you can still use it to extend the wireless network in normal range extender mode, but it just cannot join the same mesh network with Deco. You can try to change the wireless name of the RE330 to be the same as on the Deco, then check if you can roam between different networks properly.