Could a fiberglass repair kit work? I know little about the properties of fiberglass, but do know that the adhesive/bonding agent used seems to stick to a lot of things. Another possibility might be Bondo... Whatever you get to stick, if you patch both inside and outside the tank, it might hold better. You would then have to let any solvents outgas from the inside of the tank before you put your plants beck in it. A fairly thick layer of silicone calking over the whole thing might help both keep the water inside and shield the patching material from the water. Or would a patch made up of a thick layer of silicone calking inside and outside hold well enough to be a patch by itself? As you can gather, I've not repaired a Rubbermaid tank, but have a few thoughts as to how to do it, which might or might not work.

For some reason the picture isn't showing for me but I'll answer the best I can. Outside of disassembling and removing the part for a proper reglue, the most professional repair would involve pressing wedges of matching wood into the crack with glue. The grain needs to have the same orientation so it could be colored to match the original wood. That's not an easy task for someone without a lot of finishing experience but it's doable.

Would suggest filling crack w/ ridged foam board before using the calking material of choice. The calk goo will just ooze to the bottom of the void (w/o this or a similar step) and result in a big drippy mess as well as a waste of your $$.

Sorry here are the repairs