chan_pjsip, which is the SIP driver you should be using, allows you to specify a external address for each each trunk, because it allows multiple transports.
Unfortunately, as of:
there was no way of doing this in the GUI. As the online user guide is stuck and FreePBX 13, and the resolution of the feature request, mentioned in the above thread, isn’t clear, I’m not sure if any support has been added since, although I suspect the GUI support may only be for associating the custom transport with the trunk and not defining it in the first place.
Which country is it that is nobbling the internet in this way?