Exchange 2019 in Intranet?

Hello, I hope you can help me!!!
I have an intranet with two servers, Windows Server 2019 as DC and on another server an Exchange 2019. The problem I am encountering is that if I remove the router that acts as a gateway, the Outlook 2019 that connect via RDP for each user stop working. In this environment we only want it to work on the intranet.
I have checked the DNS, the nslookup resolves correctly, I have ruled out firewall issues, ping, etc...
If I connect the router, even if I don't have internet, the outlooks open and go slowly, for example making changes to the calendar.