It would be nice if dynamic DNS in gl-inet routers could allow users to choose an interfaced used for updating the DNS record from the GL GUI. As I understand, right now the router uses the default route to determine the default gateway which may not be always what is really needed.
As a temporary workaround, to make sure that the DNS record always points to WAN IP address (for example) I use curl as a follows:
/usr/bin/curl --interface eth0 https://freedns.afraid.org/dynamic/update.php?ZZZ
as a cron job.