@RTID
Hi, welcome to the community.
I used to see one similar case on Deco X50 but the user here did not mention a firmware upgrade.
https://community.tp-link.com/en/home/forum/topic/617930
Could you please help check the following details:
- Were the other two satellites Deco M9 Plus still solid green after the firmware upgrade?
- Can you still get internet when connecting to satellite M9 Plus?
After that, please refer to this link to help me submit the Deco APP logs:
How to submit Deco APP log
Then, Here are some suggestions for your reference:
1. Change the Wi-Fi SSID back to what it used to be.
2. Reset one of the satellite M9 plus by holding the reset pinhole for 1s and manually add it on the Deco APP.
How to add an additional unit to your Deco network
Wait for your reply.
Best regards.