Hi,
I would recommend logging in to your TRB via CLI or SSH interface and enter command:
uci show | grep 'fota'
Then you will receive all fota variables that you can change via uci command.
For example, in RUTX router it looks like this:
uci set rut_fota.config.enabled='0'
And to save your changes do not forget to use another command:
uci commit
Regards.