FWIW, there are two sets of two drain plug holes in the case to allow for either vertical or horizontal airflow. Since yours (should) be configured for upflow, I think they're using the wrong drain plugs, and the coil may not have been installed properly for your application. This would make the pan inoperable if it isn't installed properly as well.
As pointed out by JAD, the plug right above the pipe is a secondary drain...which should NOT be plugged. It is meant to act as a tell-tale if the primary drain is clogged, to prevent the drain pan overflow which apparently you are experiencing.
I think in his photo, "UP" is on the LEFT, so I interpret that he is using the right set of holes, just should open up the secondary port.
This is awkward, but...
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