Hello,
Thank you for your feedback.
CBC is older and have more compatibility with other IoT devices and overall historical reasons. There are performance advantages also.
Currently it would seem that additional development need to be done to router's firmware in order to support "aes-256-gcm" at the moment we support "aes-256-cbc" and others.
For development possibility Please contact Teltonika sales manager to continue on this query.
Best regards