In a nutshell...
Set up the 3102 with a fixed IP on your LAN, make note of the IP, and port number in the PSTN tab.
Create a new trunk in 3CX, see the 3CX guide in what settings to put in, use the IP you gave the 3102 and use the SAME port as the PSTN portion. The 3102 is really two devices that share the same IP, there are two port numbers, one for the PSTN and one for Line 1, the ATA part
See if the 3102 trunk is registering with 3CX.
See other posts on the 3102 in the 3CX Phone system-General Forum for tips on solving some of the problems that other people have encountered with the 3102. There is (was) some incorrect in the 3CX guide info about putting a * in the caller ID setting of the 3102.