Access Rest Webservice behind NAT

Access Rest Webservice behind NAT

In my network i have a network device (c++) which provides a simple REST
webservice. I want the webservice be accessible from outside the networ,
e.g. the internet. A PC in the internet which is not behind a NAT should
be able to call the webservice on the device. So communicating with the PC
in the internet is easy because I know the IP and Port of the PC. The
other side is not easy, because the network device is behind NAT.
How can i easily solve this? Since the communication should be
instantiated from the network device, i would first make make a call the
the PC and somehow keep the connection open, so that the PC can make calls
to the Network Device.....?!?!
What would you suggest?
kind regards