There are multiple ways to trigger CTR+ALT+DEL over an RDP session.
Open the Windows Run box by right clicking on the Windows icon and selecting run or by pressing Win+r and running the command below.
C:\Windows\explorer.exe shell:::{2559a1f2-21d7-11d4-bdaf-00c04f60b9f0}
This command could also be run in PowerShell
Another way to press Ctr+Alt+Del in a RDP session is to use the key combination Ctr+Alt+End.