How? I’m not saying this is great practice — there are certainly better options — but no one outside your network will ever know about it. It also won’t matter if you switch ISPs.
It's not a problem (well, most of the time), but you would see the requests for 'internal' resources in DNS (ie your machine is not on your network but tries to resolve the internal DNS records) and in certificate checks even for non-public PKIs
How? I’m not saying this is great practice — there are certainly better options — but no one outside your network will ever know about it. It also won’t matter if you switch ISPs.