Wrong forum. If the firewalls are simple firewalls, you don’t need anything. If both firewalls do NAT, you need to operate a VPN. If the right hand one is NAT, but the left hand isn’t, this is a standard Asterisk inside NAT configuration and you can use STUN, but you can also configure localnet and externip, instead (you will need to do this for the VPN case as well). If left hand is NAT, but the right hand isn’t, VPN would be one option. You could also treat this as an Asterisk outside NAT configuration, but you may need a certain level of NAT support in the phone, or the SIP NAT support in the left hand firewall.
A VPN solution is likely to be the easiest.
I’m not familiar enough with TURN, ICE and SBC to be able to decipher them. I don’t think they mean SMTP TURN, In Circuit Emulator and Single Board Computer, here .
The forum should be Asterisk Support, or one for your GUI, depending on whether or not you are directly manipulating the configuration.