Done. let me know if I missed anything.
Oh absolutely - you can zip or rar files into multiple archive files (which still open as a single unit).
In winrar for example you simply select that in the options, and can set it as default too.
Or if you prefer an automatic solution - rclone has a "chunker" backend that can split files for you as you desire - an a completely transparent way (so they appear as single files in the remote and function normally).
But I would not discount the ISP doing their job out of hand (assuming that it's not just a temporary thing from Corona). ISP admins are humans too - and they miss things, or automatic systems fail to detect problems. You'd be surprised at how many problems rely on user-reporting to be fixed quickly.
This is often a problem that a higher-level admin can fix remotely, so don't just assume the problem is "too big" for them to consider helping you. The worst that can happen is that nothing happens and you wasted a few minutes writing them the email 
I wouldn't bother contacting them without concrete data though. For that we need a tracert and maybe some simple pinglogs (I can help you gather that and also analyze the results) - but I'd need to know from NCW how to grab the relevant IP first, as that's not something I've had a need to do yet.