Are you sure it is MBR and not fix EFI Boot files
Use the install media or
How to Fix MBR Without Windows 7/8/10 Installation CD, DVD or USB Disk
If you have a Windows installation disk, such as CD, DVD, and USB flash drive, fix MBR by doing these operations: choose "Repair your computer" or similar feature and run "Startup Repair". Or execute commands in Command Prompt.
bootrec /fixmbr,
bootrec /fixboot
and
bootrec /rebuildbcd
How to Fix MBR Without Windows 7/8/10 Installation Disk
Download Partition Wizard Pro, register it and then use it
create a bootable disk of Partition wizard
boot pc from bootable media
select your disk needing fixed and select Rebuild MBR and Apply.
close out partition wizard
remove DV/CD or USB device, restart and all is fixed