That moment in time where you realize that you must constrain the support people from doing anything other than what you direct them to do

This is Comcast. And my internet connection in my home office. Cable modem spontaneously started rebooting on me over the last few months. Looks like it happened after they replaced my older cable modem which was working nicely, with the new one … which isn’t.
First call in this week, after it kicked out a whole bunch of times while I was working on customer machines with hard deadlines to get things done in … they scheduled a tech, after I requested a replacement cable modem. They promised/swore he would have one with him, and would replace it.
Instead, he blamed filters outside the house (that Comcast had installed previously), that he removed.
This morning while working on a machine in the UK, and this afternoon while working on a machine in Ohio, it kicked out on me. Again, with hard timing deadlines (one was a bank, another a genomics medical site) on me to get it done.
Fed up, I called them back. On the phone now. Will insist they simply replace the box. They seem to get that this is an issue. Will see if they actually do this correctly.