HTTP 503 error trying to access AD FS from the domain

Question

I'm using VMs to test my AD FS multifactor authentication plugin. I'm referring to this guide as this is the only guide on this subject. I might be doing something wrong or missing some obvious faults because this is my first time hearing of AD and its functionality, and I never used Windows Server to this date.

I successfully set up a VM #1 containing Windows Server 2019, AD DS, AD FS and DNS Server. I added the certificate for AD FS domain (fs.prj.local) and my auth plugin. I enabled the IDPInitiatedLogon page through the PowerShell. Regular auth using this page works perfectly using Forms Authentication as primary, though I can't test my plugin because this multifactor authentication method is not available from inside the VM #1 (it skips the plugin after the Forms auth), and internal access requires to specify the so-called relying party that I do not fully understand. Additionally I tried to set my plugin as a primary auth method and it works as intended.

With this info I set up a VM #2 using Windows Server 2019 that is bound to my domain successfully (prj.local) and is using the DNS server from the VM #1 that has the A record of fs.prj.local pointing to VM #1 IP address.

The only thing I can see from the VM #2 is the default webpage when sending a GET to fs.prj.local, that might be coming from IIS. Tring to access any endpoint of AD FS is returning HTTP 503 error. All the necessary endpoints are enabled in AD FS configuration.

The task at hand is to write a plugin for AD FS that enables MFA through our backend and mobile app. Using it as primary for testing is not suitable at all, because the client infrastructure already has AD FS fully set up and working, and external access for their AD FS log-in (OAuth 2.0) page is easily reachable from anywhere.

Spent almost 12 hours to set everything up with no prior knowledge of anything just to get the 503, so I'm asking for help now. Thanks in advance!