@Simone_TP 

Where did you see this message which shows can't add other devices? Could you please show us a screenshot of it? 

Theoretically, we can add multiple extender into the onemesh network system, while only one onemesh router can exist.

May it help and have a good day. 

@Simone_TP 

Hello, we did a test at our lab, the current issue is normal. The message you saw means that there is no more onemesh devices to be added.

Below is a picture in English version which is much easier to understand. 

From the picture you attached, the RE300 has been added into the onemesh network, both work properly. You can be assured of it.

May it help and have a good day.