NDES Server - works with "localhost", but fails to authenticate with FQDN

It's the first time I'm setting up a CA in combination with NDES.

I am trying to set up SCEP in JAMF. I've checked the security settings on the template and made sure the template I want to use is in the MSCEP registry entry on the NDES server.

I've set up my CA and NDES servers, and everything seems to be going well. I'm able to authenticate to https://localhost/certsrv/mscep_admin and obtain the thumbprint and code for SCEP set up, however, whenever I access the mscep_admin site through the Entra Private Connector App, I also get the login window, but when I enter my credentials, it just shows the login window again, each time. I've checked the credentials, and I'm 100% sure they are correct.

I got a little further now, on the server itself, when accessing it through FQDN, it seems to work now, but only on Firefox, so not on Edge, there I also get the login window each time.

I've run Microsoft's NDES configuration validation script, as well. Everything's come back working, except for Intune specific things (such as NDESPolicy module registry entry).

Has anyone here run into this before, or can just offer some insight?