nmkaufman's post in Changed ISP, can't access Unraid Server was marked as the answer
Bridge mode, more than likely? This disables the router in the the modem, and let's you use your own.
Go to google on your home network, and ask 'what's my ip?' Try connecting to that new IP address.
If it doesn't work, you'll need to re-enable bridge mode on the new gateway.
If you can't do that (more and more ISP's are disabling bridge mode) you'll need to use Tailscale, which luckily has never been easier.