So my UPS is a black ADR (I don't know the model number as I've lost the stuff that came with it like the box and guide, it's some chinese crap.)
Router isn't plugged into UPS.
This is what's plugged into UPS: monitor, keyboard, mouse & main PC case ofc.
No, the power doesn't cut and it still clicks, it happens randomly.
It's not an APFC PSU, nor is it modified.
Also I'd like to point out that the problem only happens to this desktop PC (connected via an ethernet cable) and plugged into UPS. It does not happen to other devices, such WiFi ones.
I'd bypass the UPS and connect direct to the power mains to test things out.
If you get clickity-clicks from the UPS, with nothing connected to it, and the network connection remains stable; or the UPS stops doing the clicking thing altogether, then you have found your problem.