Glues will only work with lots of contact area, and that area needs to
be in the sheer plane, not effectively a butt joint.

Polyurethane glues once foamed up have much reduced strength - it's
only in the areas tightly clamped up that they offer strength.

Epoxies will gap fill *and* be strong - but it still doesn't sound
like the contact area is sufficient.

Fillers have no significant adhesive properties.

External steel angle brackets might be a viable solution.

Otherwise slice off the mess and either replace the whole thing or
properly joint on new timber.