You explain why it is important to notify about their EOL modems, but you fail to explain why this, of all options, is the appropriate communication channel.

At the very least, you have customer addresses. You should also have phone numbers and email addresses. If you have a way to bill customers, you have a way to contact them.

Injecting JS into HTTP sites is disgusting. It violates both the user's and the site's expectations and is entirely unnecessary.