----Edit----
My mate got back to me. The issue was the default priority of the network adapters, like jamvanderloeff said in his comment, windows assumes ethernet is fast, and by default will give it a lower priority (meaning more likely to use) whether it has connectivity or not. Changing the priority numbers of the two connections solved the issue.

https://techit-services.com/set-the-priority-of-network-adapters/

Instructions to fix came from here

----End Edit----

I can't figure out how to google this, so here I am.

We had a storm that knocked out our broadband, normally my PC is connected to the network via ethernet to the router, I have a NAS on the network with a lot of media on it that I'm accessing to pass the time.

I'm using my phone as a USB hotspot in the meantime for small google searches and socials because that's all I've got enough reception for (Beyond the black stump, 1 bar max reception, no video streaming here).

So windows reports two network adapters, the ethernet with no internet connection, and the phone via USB with internet access. If I try to load a web page with the ethernet plugged in, the page will load VERY slowly if at all, and usually it times out, unplugging the Ethernet resolves this and allows me to load webpages, but then accessing the NAS becomes impossible, for obvious reasons.

I can't work out why this is happening, I don't know how to google this issue to solve it, I don't even understand why this is an issue in the first place. I would've thought Windows 10 being the advanced piece of software it is would be intelligent enough to route internet requests to the adapter that has internet available and not to the adapter that has none. It's really confusing, frustrating, and I'm at the end of my tether. No pun.

I'm hoping someone can help me. Cheers