Inconsistent upload speed along various applications?

Since the last few days of august I had a quarter, sometimes even an eighth of my normal upload speed of 2 mbps. That time I thought it was something to do with my ISP's Fair Usage Policy which slows down your internet after you hit a certain quota. But then august turned into September, and my download speeds were back to normal, yet the upload speed was still very slow.

On online games I also suffered packet loss I have never experienced before then. I did not change anything on my router nor any of my network settings on my PC but the DNS (from Google's to Cloudflare's), but after reverting it still proved fruitless.

Oddly enough, on 1 app (Discord Canary) my upload speed was normal, 2 mbps. Yet on all of the browsers I tried (Firefox, Chrome, and Edge), they all had 0,2mbps to 1mbps upload speed. On all of the apps but Discord Canary I also had under 1mbps upload speed (even normal Discord and Discord PTB). I tried to bear the slow upload but after about 5-10 minutes my ethernet shortly disconnects and reconnects, interrupting the upload. The problem also happens during a video conference, requiring me to disable and reenable the ethernet adapter, or replugging the ethernet cable; obviously a great annoyance since I sometimes miss out on important information.

Theres 2 questions I want to ask on this sub, is the problem on my side or the ISP's, and if it's on my side, what could I do to fix the issue?