Recovering from Invalid Container error in APFS volume (macOS)

Question

I have an external disk formatted in APFS. I can see the disk and partitions, but macOS is not mounting the volumen and disktuil and fsck are giving me lots of errors.

Any help recovering the containers would be really appreciated.

Here's the results from disktuil, fsck_apfs and gpt

diskutil list
/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk4
   1:                        EFI EFI                     209.7 MB   disk4s1
   2:                 Apple_APFS                         499.9 GB   disk4s2

sudo fsck_apfs -q /dev/disk4
0000:  0000 0000 0000 0000 0000 0000 0000 0000       |................|
. . .
01c0:  0200 eeff ffff 0100 0000 2f60 383a 0000       |............8...|
01d0:  0000 0000 0000 0000 0000 0000 0000 0000       |................|
. . .
01f0:  0000 0000 0000 0000 0000 0000 0000 55aa       |..............U.|
0200:  4546 4920 5041 5254 0000 0100 5c00 0000       |EFI.PART........|
0210:  1eb6 89e8 0000 0000 0100 0000 0000 0000       |................|
0220:  2f60 383a 0000 0000 2200 0000 0000 0000       |..8.............|
0230:  0e60 383a 0000 0000 cad6 40ec 1c52 6541       |..8..........ReA|
0240:  8b8a a0b8 aa5f 119a 0200 0000 0000 0000       |................|
0250:  8000 0000 8000 0000 bb88 aed4 0000 0000       |................|
0260:  0000 0000 0000 0000 0000 0000 0000 0000       |................|
. . .
0400:  2873 2ac1 1ff8 d211 ba4b 00a0 c93e c93b       |.s.......K......|
0410:  2229 38e8 e2aa 2f42 9bd7 a515 3018 68b0       |..8....B....0.h.|
0420:  2800 0000 0000 0000 2740 0600 0000 0000       |................|
0430:  0000 0000 0000 0000 4500 4600 4900 2000       |........E.F.I...|
0440:  5300 7900 7300 7400 6500 6d00 2000 5000       |S.y.s.t.e.m...P.|
0450:  6100 7200 7400 6900 7400 6900 6f00 6e00       |a.r.t.i.t.i.o.n.|
0460:  0000 0000 0000 0000 0000 0000 0000 0000       |................|
. . .
0480:  ef57 347c 0000 aa11 aa11 0030 6543 ecac       |.W4........0eC..|
0490:  45a5 83d7 44b2 4145 a88b b2e8 a946 8e59       |E...D.AE.....F.Y|
04a0:  2840 0600 0000 0000 0760 383a 0000 0000       |..........8.....|
04b0:  0000 0000 0000 0000 4500 7800 7400 5f00       |........E.x.t...|
04c0:  4800 4400 0000 0000 0000 0000 0000 0000       |H.D.............|
04d0:  0000 0000 0000 0000 0000 0000 0000 0000       |................|
. . .
0ff0:  0000 0000 0000 0000 0000 0000 0000 0000       |................|

error: Device does not contain a valid APFS container.
   Container superblock is invalid.
** QUICKCHECK ONLY; FILESYSTEM DIRTY


sudo gpt show -l /dev/disk4       
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - "EFI System Partition"
     409640  976363488      2  GPT part - "Ext_HD"
  976773128          7         
  976773135         32         Sec GPT table
  976773167          1         Sec GPT header