Subscription1 contains VM1. From your Windows 10 PC (Computer1) on the Internet you added a network interface vm1173 to VM1. RDP to VM1 fails. To enable Remote Desktop access to VM1, what should you do first?

The most immediate and fundamental reason you cannot RDP to VM1 is that it is likely in a stopped or deallocated state. A virtual machine must be running to accept RDP connections. Starting the VM is the first logical step to enable access. Changing the priority of an RDP rule might be necessary if there's a conflicting rule, but it's irrelevant if the VM isn't running. Attaching a network interface was already done (vm1173 was added), and while a VM needs a NIC, this isn't the first step if the VM is off. Deleting a DenyAllInBound rule could be a necessary network security group (NSG) configuration, but again, it's secondary to ensuring the VM is operational.