MacOS partition shows up as FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF (Mojave)
This solution will work because you already have System Integrity Protection (SIP) disabled.


  Note: The current version of gdisk is version 1.0.4. This is the only version that will recognize Apple's new APFS format. So, if you already have an older version of gdisk already installed, then you will need to download and reinstall before proceeding.


The gdisk package can be downloaded by clicking on this link.
This should place the file gdisk-1.0.4.pkg in your Downloads folder. Before installing this package, enter the command given below in a Terminal application window to remove the quarantine attribute.

xattr -d com.apple.quarantine ~/Downloads/gdisk-1.0.4.pkg


Once gdisk is installed, you will need to enter the commands given below.


  Note: The sudo gdisk /dev/disk0 command may ask for your password.


sudo gdisk /dev/disk0
t
2
AF0A
w
y


An example is given below.

Marlin:startup davidanderson$ sudo gdisk /dev/disk0
Password:
GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): t
Partition number (1-3): 2
Current type is 'Unknown'
Hex code or GUID (L to show codes, Enter = AF00): AF0A
Changed type of partition to 'Apple APFS'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk0.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.
Marlin:startup davidanderson$ 


After restarting your Mac, you will need to enter the following command to reclaim the free space back to macOS.

sudo diskutil apfs resizeContainer disk0s2 0


When finished, you should enter the following command to enable SIP on your next restart.

sudo csrutil clear