Worsening internet connection and infuriating customer support - my work from home job is now at risk and I still can't get any help

Over the past two months, my internet connection has steadily deteriorated. The connection seems to randomly drop and reconnect. It is becoming much more frequent as time passes. When it's actually working it works as expected, but it's rare this lasts more than ten minutes at a time now. It took me a full minute to simply load this web page so I could begin typing up this forum post, and I have now completely abandoned any hope of successfully watching livestreams, joining online calls with my friends, playing online games, or anything of the sort due to the constant disconnections and instability. This was never a problem until about two months ago, and since then I've had countless calls with customer support and two techs have come out to my home but none of them were particularly helpful or thorough.

I am not the most tech savvy person, but I have done my due diligence and my best effort to rule out everything I can. This is a list of changes made and attempts to diagnose the problem:

• Replacing every cable from the modem to the walls and from the walls to my devices. No change. I can't do anything about the cables in the walls, but I am doubtful that the problem lies there and has somehow affected every individual cable to every room in the house starting all at exactly the same time.
• I ran a lengthy extension cord in from another room and tried four separate outlets from four separate rooms in my house after one customer support agent insisted to me that it was a problem with the modem not receiving enough power. I even used different extension cords to rule out a possible faulty extension cord. No change at all.
• I disconnected every single device from the network, both wireless and wired. Even with only one desktop computer connected, problems persisted. I carefully added each device back slowly, one at a time, just in case. No change.
• I tracked total device usage in the house and tried to see if it corresponded with the down times - it does not.
• I have restarted my modem almost thirty times now. This has caused more down time than it has prevented.
• The second tech to come to my home replaced my modem completely, and the problems have persisted exactly as they did before.
• I've had six different customer support agents tell me that they are sending "correcting signals" to my modem. I have no idea what this means, but it has never helped.
• I have updated software and drivers and ran full system virus scans on every single device connected to the network to be absolutely sure it is not a problem caused by a device of mine. No change.

I've used online ping tools, and I've found that seemingly at random a ping attempt will take anywhere from ten to one hundred times longer than the average result, or simply completely fail for long enough that the website stops working.

Weirdly, I've noticed when in online calls, I can frequently hear other people in the call, but they cannot hear me. Like the download still works but upload cuts out completely. This happens constantly, and a complete and total connection drop usually follows these.

Explaining the issue and my attempts to resolve it has gotten me nowhere at all with customer support. I do not believe that restarting my modem for the thirtieth time will do anything for me now, and customer support now has me trapped in a loop with them where they restart my modem and then tell me that it all looks fine now, and now matter how much I tell them that this wasn't my problem, I get nowhere else with them. Sure, it works fine briefly again after the modem is reset, but within half an hour it will be right back to the usual problem. For some reason I cannot get them to understand that, and I've been unable to get anyone who is able to do any kind of further assistance to help me try and diagnose this issue.

It has gotten so bad that it is affecting my livelihood, as I work from home and now am frequently unable to do my job due to these problems. My work requires a steady connection for security reasons, and it has been made clear to me that I will lose my job if I do not have internet capable of allowing me to perform the work. Seeing as how this wasn't a problem at all two months ago, and I'm getting the cold shoulder from xfinity, I am starting to feel desperate.

Any help at all or any kind of direction from anyone would be deeply appreciated. I am at an absolute loss on what to try next or who to ask for help. I can't lose my job. I've already lost the ability to talk to my long-distance friends or enjoy any of my gaming/streaming hobbies, and losing my income too is going to possibly literally kill me. Help me, please.