No Internet Access (DNS Issue?)

Let me start from the beginning. About a week and half to two weeks ago, my internet seemed to be getting slow because youtube videos would automatically change to the lowest resolutions, and when streaming movies every once and awhile the quality would drop then go back to HD. I didn't think much of it so I did not do a speed test. Fast forward to two nights ago. Right around 8:30-9pm, I lose internet on all devices. To be more specific, we were still connected to the WiFi, but just had no internet service. Usually this would resolve itself after a few minutes and the internet would come back. This time it didn't. I went onto the xfinity app and was still able to restart the gateway from my phone probably because I was connected via wifi even though I had no internet. It restarts, everything seems to be back online, and we go to bed. In my home office, I have two PCs hardwired into my xfinity gateway, one is my person gaming PC and the other is my work PC. For my work PC the ethernet goes from the gateway to a IP desk phone then to the work PC. The next morning (Yesterday morning) I hop on my work computer (Note: My work PC does not connect to a VPN, so it acts just like a normal personal PC) and no internet access on neither the PC or desk phone. I check my Gaming PC, I have internet. Both are connected to the same gateway via ethernet. Very weird. I restart both computer. Same results. I restart the gateway, same results. Directly connect my work PC to the gateway, same results. tried different cables, same results. So on my gaming PC I start researching and found out the most common solution is to manually change the DNS address on the PC to google's 8.8.8.8 & 8.8.4.4. I did just that and BOOM!, internet restored on my work PC. But my desk phone still has no service because its programmed for the DNS address 75.75.75.75 (xfinity's DNS address) and I can't change it. So my desk phone is still dead in the water at this moment. Ok, next I did an experiment, that I would soon after regret doing, on my Gaming PC. Remember this PC had no issues and had internet while on the default DNS address setting "obtain DNS server address automatically." I manually changed the DNS server addresses on my Gaming PC to google's, still had internet. Then I changed the setting back to "obtain DNS server address automatically." After I did that, my Gaming PC ended up exactly like my work PC, "No Internet." I contact Xfinity support, and they tell me I have too many devices connected to my WiFi. one of the people I talked to even said it was "illegal" to have that many devices connected to my gateway with the internet speed that I'm paying for, and that I need to upgrade my internet plan to a higher speed. As soon as I heard that I got off the phone with them immediately because I know that was a load of [Edited: "Language"]. At some point I decided to restart the gateway again but by unplugging it from its power and waiting 30 seconds. It boots back up, get the same issue on my two PCs, but now I notice one of my ring cameras are disconnected and our fancy LitterRobot was offline too. Later that night, I restart the gateway again, my ring camera gets back online, but the LitterRobot is still offline. I'm a technical guy. I may not have any background in networking, but I have an above average understanding about computers and the internet and I do my research and I basically do troubleshooting for a living. I have a 300mbsp service, and I know for a fact that speed is perfect for me and my household because I had all my devices connected to it, including ring cameras, smart home plugs and switches, google homes, etc. and I would do speed tests and get higher speeds than what I'm paying for. My internet has been working flawlessly since I got it in 2020 up until 2 days ago.  I spoke to another person in technical support, and luckily they reassured me that my gateway was working and my internet service is working perfectly even with all the devices connected to it (about 28 devices). But he could not solve why my PCs and other devices don't have access to the internet via the xfinity DNS servers (75.75.75.75). I'm going to get my gateway replaced later today, and see if that will do anything (I doubt it). But after all my troubleshooting and research, the conclusion that I can come up with is that there is something wrong with Xfinity's DNS sever address 75.75.75.75 and 75.75.76.76. I don't know why some devices can get online and some can't, No xfinity representative knows either. I'm at a loss here. If anyone else had a similar issue, and found a solution, please let me know. Thank you! But in the meantime just ot get my work done, i have to use google's DNS server addresses.