Crack in Cast Oven after Curing

[Dosache]

I have a modular kit oven that I was finally able to cure. I had 5 days of successively hotter fires and closely followed the manufacturers instructions. The oven had 1" of rock wool over the entire outside except for the opening/flue. The morning after my last night if curing I woke to find this crack that totally bisects the monolithic entry piece. I have talked to the manufacturer and they say I should have insulated the entire oven prior to firing, though that's not clear in the instructions. That being said I need to repair this and figured who else to ask rather than the FB experts. Any tips or tricks?