If you are getting disconnected from the game or can't login with no error messages or a message indicating an issue with the connection, usually this is caused by a temporary issue with your internet. If this problem persists, you can try restarting your modem and device.
If the issue persists, test using a different network in this device or a different device in the same network and see which one works.
If you notice that the issue is on your internet, you can try again later or contact your provider.
If the issue seems to be in your device, it could be related to a firewall or antivirus, in that case you can try adding PokeMMO to an exception list in it.