1 - Go to Advanced feature and turn on Zepo Mobile site option.
Please find the whole process for Big-Rock/MITSU/DOMAINS N MORE below:
Select My account
1.Go to Manage orders> list/search order
2. Select DNS Management> Manage DNS
This m site would be mobile responsive and would be based on Layout 5 Foppish theme.
Once domain is active and if anybody opens up the store on mobile as your-custom-domain.com, then it would redirect to m.your-custom-domain.com automatically.