Filler won’t work & the cracks will almost certainly reappear. You need to add additional screws into the joist either side of the crack to stiffen it all up, apply some fibre glass reinforcing tape over the crack (I use 3 slightly overlapping strips) & get a plasterer in to skim over & blend in with the surrounding ceiling. Even this will not guarantee the cracks won’t appear again if you’ve got undue flexing in the joists above.
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