Hi Asfand,
I think the Cellurar is only for failover. If you don't have any kind of WAN connected I'm not sure if it's works or not. I couldn't test it yet. Maybe @PhilipDAth has more knowledge if this could work or not.
One thing how you can overcome this issue would be to buy a 2nd router and connect it before the Meraki. In this way you will have a portable antenna what you can place to the window for better coverage.
Hopefully this helped.
Cheers,
Greg