Okay I have figured out the issue. Don’t know exactly down to the technical aspects but basically My WiFi runs in 2.4GHz (802.11n only mode). If I am close to the router I get full speeds. Wheras if I’m away from the router, the speeds negotiated by my devices and the router get a 50% cut. I can see the negotiated max speeds getting lowered by 50% when I’m far from the router by logging into its UI. The funny thing is only the upload speeds take that 50% cut. Download speeds are unaffected.
As for my previous theory that the IP subnet might be the issue: each time I reset the router, I was physically close to the router and my devices registered a full bandwidth. So that was the confounding factor in the equation.