Can you connect to your Nextcloud domain in a web browser from your Windows tablet straight up, no Cloudflare or anything else in-between?
If your domain resolves to your public IP address and you’re on the same 192.168.x.x private LAN with the server, then most modern ISP routers will block this. I think this used to be called NAT Reflection or Hairpin Routing or something like that and I’m not sure why it used to work, and it may even still work for some, but it never worked for me and it’s a common issue when self-hosting.
It’s also very inefficient and you would find it very slow since your traffic would have to go out to the world and then come back to your home server using your external bandwidth rather than connecting directly over your LAN.
Your phone is probably accessing your domain via its external, cellular interface (public IP), so it’s outside of your local network when it does that.
Refer to this thread as it might be similiar.