Intermittent connection to mail server.

Hi, I recently switch to new mail host. We used both Mac and Window Outlook to connect to the mail server. 

We realised intermittent connection and disconnection. If i switch my connection from network to 4G, i will be able to connect to mail server.

I'm not sure if i need to do something on the firewall to make sure I can access to the mail server throughout without error and switching to 4G on my mobile.

Thanks