That is one killer feature request that I wish could be implemented within the easy-to-use GL.iNet UI. Others here have suggested that it is indeed possible but would need way too much tinkering with advanced settings and command lines that are way beyond me.
Was it not the case that the failover actually failed to kick off unless the cable WAN was physically disconnected from the port and was not taking over merely with the internet failure or has this been fixed since?
You do not need to detach the cable to make failover work. The router will detect if there is no Internet in one connection then start failover. This happens much slower than just disconnect the cable.