If the issue only started after you upgraded your internet connection, and nothing else changed on your computer, and you are using the same exact mIRC installation as before, the most likely reason is that your internet provider is checking for IRC connections on your new line and is disconnecting them.

mIRC is the most widely used IRC client, so internet providers will almost always check for connections that look like "mIRC". All IRC clients identify themselves in some way, whether it is with the VERSION reply, or other variations in the way they use the protocol.

mIRC sends pings specifically to detect whether your connection has failed. If mIRC sends a ping and you see 10053, this means that your connection had already failed and mIRC detected it.

The only way to rule out your internet provider as the cause is to follow the advice that I provided in my previous email. You will need to 1) contact your internet provider, and 2) try connecting through a VPN. A VPN allows you to make secure connections that your internet provider cannot interfere with.