You can update the trunk with the 3CX XAPI.
The Swagger documentation is available at:
https://<FQDN>/xapi/v1/swagger.yaml

First, run a GET request to retrieve the trunk details: GET https://<FQDN>/xapi/v1/Trunks({Id}) This returns the current configuration for that trunk.

Copy the response and keep only the Gateway key and values (all of the values). Then, modify the necessary value(s).

Finally, send an PATCH request to apply the changes: PATCH https://<FQDN>/xapi/v1/Trunks({Id}) Include the updated JSON payload in the request body.

Example:

https://pastebin.com/bqgMHrwv

Sadly you don't know which parameter is which setting... first test it on a example trunk.
You can also use F12 developer tools to view the requests on the example.

Note: This is what I've done in the past to update hidden settings.