Lost wireless (Wi-Fi or Cell) coverage while connecting or while connected
Verify that you have either Wi-Fi or cell phone access before you try to connect to the remote computer.
On a cell network, a break in the communication link, such as moving through a cell dead zone, switching cell towers, roaming, and other issues, may cause the RDP connection to drop.
On a wireless network, turning on a microwave oven, your house cordless phone ringing, and many other wireless devices, can create enough noise to interfere with your mobile device wireless communication, causing the RDP connection to drop.
To correct this problem, move to a different location if possible, and avoid interference sources.
Connection to server cannot be established using a Manually created connection
Verify that the computer and mobile device you are trying to connect are on the same network or that port-forwarding is enabled on your router.
The certificate was issued locally by your computer or by a non-recognized certificate authority.
Check the certificate of the remote computer, or if you trust the remote computer, on your iOS device:
Go to Settings
Turn ON the “Accept All Certificates” option.
Using VPN and Manual RDP Connection
Verify that the remote computer can be reached from your your mobile device. Try using a "Ping" utility to verify.