hello :
if you only have a MCU , you can configure MCU as firewall mode ,then the two network can connect via the MCU
and if you have one or two SC , you can configure the Traversal between public and private networks( please confirm there is Traversal Traffic license)
configure the NAT or two network segment for SC ,register two network terminal to SC
then the two network terminal register to SC , then you can make call use their register ID(SIP or H.323)