Help using RDP via a tether
The main problem of RDP is that you need a public IP address for directly establishing the RDP connection on port 3389.
In your case there are two obstacles which make it next to impossible:

Nearly all mobile data plans do not grant you a public IP address. Instead you are getting a private IP address from a carrier internal network. To access servers on the Internet this network uses carrier-grade-NAT (NAT on carrier side, this implies that you can not configure a port mapping as you can do on DSL home routers which also do NAT).
The main reason for most carriers (nowadays) is that they don't have enough public IPv4 addresses to assign every device an own. And mobile networks providing IPv6 services (und may be a public IPv6 address) may exists but I have not yet encounter one.
Some carriers allows communication within this private network, in such a case you could use a second cell phone for the RDP client and then connect to the carrier network internal IP address of RDP server. The problem is how to get this IP - regular dynDNS clients I know only give you the public IP.

For tethering the phone acts as NAT router, therefore even if you would manage to get direct access to the IP of the cell phone attached to the RDP server you would still need to forward the RDP port from the phone to the PC.