How to receive Client = Server connections using vpn software? [closed]

I have remote access software (Client / Server) and want to make the connection between the Client (connect to the Server using a host name) = > Server using vpn software on the Server side, but this is not working.

The server side looks similar to this interface:

Basedonmyresearch,youneedtoconfigurethevpnsoftwarevirtualipfortheDMZfieldonmymodem,asshownbelowinthescreenshot.ItturnsoutthatmymodemhasanipvalidatoranddoesnotallowyoutosetaclassA(vpnadapter)ip,justaCclassip.

Theaboveprocedureisstandardforthistypeofconfiguration(forClient/Serverconnections).

  

OBS:EverythingworksfinewhenDMZissettingwithip(classC)frommyadapterEthernet.

So,basedonmycase,howcanIgetclientconnections=>ServerwhenthePC(wheretheserversideis)is"accessing the internet" through the vpn software (vpn connection)?

I hope I explained my problem well: -)