Real User Monitoring auto-injection is done by OneAgent and is supported for the following technologies/servers.
The link was shared just to know the supported technologies OOB. Did you try the agentless Real User Monitoring?
Have you tried to install the oneAgent on that server? My guess is that if the underlying technology is supported by Dynatrace, for example IIS, it should work automatically.
If there is another non-supported technology, you could always monitor the user experience by using the https://www.dynatrace.com/support/help/how-to-use-dynatrace/real-user-monitoring/setup-and-configura...
So I guess something is conflicting with the automatic injection at the IIS level. Do you know if you can add any custom html/js in the screens provided to your clients? If that is the case you can add the reference to the js agent