It sounds like it has been setup, but put into diagnostic ‘dump’ mode by long pressing the infinity/join button. Here’s my suggestion:

Power off the Sub-Mini and wait 30s and then press the join button, keeping it pressed as you power it on. Do not let go the join button until you see it flashing Amber. Then let go.

After a few seconds the flashing Amber LED will switch to flashing green.

If all is well so far, open the ‘mobile’ S2 Sonos App from its fully closed state to see your original speakers and system and goto ‘Settings/System/Add Product’ and follow the on screen instructions to add the Sub-Mini to your system.

Note: your Sonos mobile controller device needs to meet the S2 App requirements shown in this link: 

https://support.sonos.com/en-us/article/sonos-app-requirements

Solved!

I reset the sub and now it’s working 

My issue also resolved after resetting the sub.