You will need a SIPdeafult.cnf file, a SEP[MAC].cnf.xml file and a SIP[MAC].cnf file and a ringlist.dat at the very least - can you itemise which files you are using on your TFTP server, as well as list examples of them?
Also, in the SIPdeafult.cnf file it is useful to define the Outbound proxy as the internal IP of your 3CX Server.
I am assuming that your internal TFTP server has been verified as working?
I would also suggest having somewhere on your internal IIS Server a dialplan.xml file as well.