Connecting two offices


i have 7 offices that i need to connect to my MAIN PBX for internal communication

my main PBX is FreePBX HA.
when i try to connect to my main asterisk server i am unable to connect to the remove offices as i am using the Virtual IP to connect to my main pbx from the countries. I ended configuring OPENVPN between countries but that is causing extreme delays on the communication.

I’ve tried IAX and SIP

any suggestion?