Server Unavailable on Android

First of all, my best wishes to the TMS team.
Thank you very much for your hard work.

I am currently testing your MQTT library.
I have created a free account with CloudMQTT for that purpose
This is working almost perfectly.

Running on Windows 10, the connection is successful at all time.
On Android, I get sometimes "Server Unavailable" error without understanding the reason.

Is there a way to make sure the connection is always successful on Android ?

Thank you in advance