Please remind that when activating mesh, the sub device should not have wan connection.
Have you got sub device shown in Main device?
This is a bug, once sub device mesh sucessfully, it should has internet led on.
When snapshot firmware fixed this I’ll let you know.
The sub device will turn off UI, so you can only access it by ssh. 
To get its ip, run command in main devce:
cat /etc/config/gl_son