Perhaps you need to contact your internet service provider.

I have been unable to connect my desktop Thunderbird to the internet as well. The phone app works well using both mobile and our home wi-fi.

I cannot save even to drafts.

I have been in touch with the internet provider. The issue doesn't seem to be at their end.

Problem on desktop. Works fine on laptop, both running Bitdefender and Bitdefender VPN, and on mobile. Completely flumuxed!

You mention two products that make my blood run cold. Mail mashers both. Disable them. Preferably by restarting your device in safe mode with networking and testing with a wired internet connection. If Thunderbird then works you have found your cause.