This is what I would try. Although I am open to correction.
Step 1. Take hold of the keel box and give it a wiggle to make sure that it is still solidly attached. If it comes off in your hand, it is better to know now.
Did it start to come away?
Yes, go to step 2. No, go to step 6.
Step 2. Remove the keel box, and any glue from the hull and other surfaces.
Step 3. Key the gluing surfaces with course sandpaper, or even a modelling knife to give the glue something to bite on. This lack of good gluing surface could have been the cause of you original problem
Step 4. Take time working out how you are going to hold the pieces in place when you glue up. You might need to make a jig.
Step 5. Using a 5 minute or longer cure resin or glue, glue the keel box back in place. The longer setting time will give you all of the time you need to put the pieces together and re-align if things shift when clamping up. If you can only use your hands, 5 mins it not too long to hold things together manually. If you are clamping, a 2 hour epoxy is stronger than a quick set (usually).
Step 6. Practice/investigate If the keel box glue is sound, then see if you can glue around it. I would not pour a lot of resin in to the hull, if I could avoid it. See if you can make an 'L' shaped spreader to reach around the front of the keel box. Or see if you can make some sort of dispenser, maybe from one of those over-sized syringes, with a bit of tube on the end; maybe even use a bent straw. Then attacking from the side, see if you can get glue to then run down the front join. You can practice first with water.
Step 7. Glue up. Using 2 hour or longer set resin, pour around the keel box. Pay attention to the date of manufacture and thoroughly mixing, to ensure the resin goes off property. If you save this job for the autumn, watch out for low temperatures which also affect setting.
Sorry if I am teaching you how to suck eggs.