recover data from FFFFF partition inside an external drive
I managed to resolve that. For those who are interested here is the solution:
Install gdisk with the following command:
brew install gdisk

After that, run gdisk passing the disk that has the FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF.
sudo gdisk /dev/disk3

Then you will be prompted with the following questions. Answer like I did below.
Command (? for help): t
Partition number (1-3): 2 // This is because the partition 2 has FFFF (see question)
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

After that your volume will not be FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF anymore and you will be able to mount it and get your files back.