Ok, so I tried, but resetting only the sub node wasn’t enough. Eventually I had to reset both main node and sub node in order for the mesh to work.
It still works a bit strange (sub node mesh light flashes slowly) but it seems that clients can connect to it.
I noticed though that not all wifi paramters were copied to sub node: On main node, main wifi ssid is hidden, but on sub node its not.
I had to manually connect to sub node (using ssh, since admin panel was not accessible) and make the wifi ssid hidden.
Also, I noticed that ssh configuration was not copied either. On main node ssh is on other port than default, and is limited to only LAN interface.
On sub node it still had all the default settings.
So in short, mesh is finally working after many trials and lots of time wasted. Still not perfectb but at least its working.
@alzhao thanks for the tip about resetting configuration. This is not documented anywhere for mesh to work. But at least others may benefit from this post.