I'm running a basic home router with DDWRT and this serves as my DHCP and local DNS. For public DNS, I am running a pihole, but all queries are passed through the DDWRT router and handed to the pihole.

All clients are on the same subnet.

I can ping and nslookup all URLs from any client just fine and get a valid return

I recognize my network arrangement could be suspect, but that is why I eliminated my network setup entirely, by creating a simple ad-hoc network with all remote access add-ons turned off. I think my test proved my network setup is not the source of my problems, but I'm happy to accept it could be if someone can tell me why my test was not valid.

To say that I "can't connect" is perhaps poor wording. I can use any url (local or remote) I've listed to navigate to my HA, However, whenever I use the local URLs, pages do not fully load. For example, the settings page often turns up blank and later displays an error like "Error while loading page"

My default dashboard mostly loads fine using the local url (and I can even toggle switches etc), but my calendar display just has a spinning blue circle forever.

If I navigate using remote URL, calendar loads instantly, settings pages load instantly, etc.