Intermittent or no internet every day

I just want to start off by saying that our internet worked 100% for the last year or almost a year on a 10Mbps line with about 4users connected so about 6 devices at a time. Last week Wednesday after the line was supposed to be upgraded, we started noticing a serious downward spiral in the performance of the line, getting anywhere between 1Mbps and 3Mbps and devices being disconnected constantly. This lasted up until Friday last week and was working perfectly fine Saturday and Sunday the entire day with speeds reaching 20Mbps. Then starting this week 7am Monday morning no wifi, then to having decent speeds, then to having 1-2Mbps download speed with all other devices being disconnected. This is still currently the issue. I've spoken to numerous call center agents that tell me to restart CPE, tell me to monitor the line with one device connected straight, telling me to upgrade my firmware. So let me tell you something I honestly can't see that its a firmware issue if the wifi/line was working perfectly for the last year. I have taken another router straight out of a sealed box and plugged it in to get the exact same issues. I have tried 2 of the same TP-link routers(Archer c20) and I have tried a dlink router and they all have the same issue and were all working 100% before line upgrade. The next thing is I can't sit and test/monitor this connection the entire day. We have 3 people here that have to work from home during this pandemic so there is no way that would work, currently, everyone is having trouble doing there work. I need you guys to sort out whatever issues were caused here and do it ASAP. We are forced to pay a service we are not receiving and this new "benefit" that we also not receiving or that we are intermittently receiving. You are my ISP who I am paying to deliver a service, but you are putting everything into my hands to sort out that is very poor service. I also know other people that are having the exact same issues and its such coincidence that the line seems to work in the evening when there is supposedly "less" congestion on the line, but drops during the day. Our wifi is a mess and I need this sorted.