Ah mate, I'm a fuckin gobshite. Someone else recommended that I set a static IP in my laptop as well as the pi box, then connect them via ethernet and see what happens..... Well that worked!
So perplexed and realising that the problem was now the router, 8 then called my ISP and spent 30 minutes of my life that I'll never get back, doing what the rep asked. I explained to her that I had performed a hard reset on the router 2 days ago (which is when the problem started) so I doubt it's as simple as rebooting the devices.
Turns out, I'm the fool that forgot to reboot the damned modem when I reset the router in order to give fresh connection to both. I restarted the modern and router and hey presto, I could connect immediately.
I felt like a dope after that, but at least I got the problem solved. Next time, I'll RTFM before I go looking for help 🤣