Cant install bootcamp in MBR after internal SSD upgrade
You could install rEFInd as by default this will automatically identify legacy Windows installations on internal or external disks.

Alternatively you could create a hybrid member on your internal SSD using gdisk and set at least one partition bootable. 

This will cause the firmware to initialize the BIOS/CSM/legacy support. 

You can pick any partitions as macOS will ignore it when booting.  If you have EFI, Macintosh HD and Recovery you could pick 2 and 3.  For example :


sudo gdisk /dev/disk0
take option r for recovery menu.
take option h to create hybrid member.  Here partitions 2 and 3 are chosen with 2 set bootable.


Type from one to three GPT partition numbers, separated by spaces, to be
added to the hybrid MBR, in sequence: 2 3
Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): Y

Creating entry for GPT partition #2 (MBR partition #2)
Enter an MBR hex code (default AF): AF
Set the bootable flag? (Y/N): Y

Creating entry for GPT partition #3 (MBR partition #3)
Enter an MBR hex code (default AF): AF
Set the bootable flag? (Y/N): N



take option w to write changes and exit.


Depending on your version of macOS you may need to disable SIP to use either gdisk or install rEFInd but you can enable it again after.