Hi @SF-6505
Unfortunutly , it's not possible to ensure the high avaibility of FSMO.
To move a FSMO role to another DC , the source and target DC must be online.
When the DC with FSMO roles crached , you have perform fsmo seizing to force the move of FSMO roles to another DC.
Before launch FSMO seizing , you have to ensure the source DC cannot be resored and cannot be connected before its rebuild.
Please don't forget to mark helpful as accepted