Connection/Delay/Hit reg

I recently upgraded my internet and hardware from 50mbps to full fibre 500mbps connected via Ethernet and at the same time I upgraded to a 4k 144hz monitor. 

At first, Going from 60hz to 120hz and the reduction in ping felt like playing a different game. It felt crisp, smooth and a lot less delay. 

It stayed like this for around 3 days. Hit reg was a lot better and one major thing was that I was not getting shot behind cover or being 1-clipped before seeing an enemy. It felt like how it should, being hit when out of cover and not being hit in cover. However, as the days have gone on, it seems to be back to the same old. As if I’m 2 seconds behind other people (especially noticeable while playing tdms and watching back killcams), enemies hitting me when their sights aren’t even fully on me (as if some kind of ping compensation or prediction issue).

Can someone please shed light on what it could be and how I can help get the game to run as it did. 

TIA

note. Ps5 - Lg 4k 144hz monitor (HDMI 2.1 /Vrr/set to 1080p/120hz ) - Sky 500mb/s Full fibre via Cat6 Ethernet cable.